Bypassing can motor control boards for use without the system if it fails is very possible; One direction=$5-ish, 4 wires. Other can motor systems could likely be swapped in too.
So normal operation would cost a bit more, but usually a board swap , maybe new mounting is all it takes. That should be within your abilities with little if any help. Most adaptions/repair diffiçulty is based on what the user expects out of it in features. The simpler ones are just that; simple. Once the track ac gets made into dc it's like a battery op. toy with boards gone; +/- power wires and a motor.
Most MTH has less of a chance at electronics surviving a child imo. Early boards fry really easy, later is better and you'll have to mind the transformer used. Good stuff, I just don't see MTH as kids stuff.
The old open frame motor has a unique feel at the throttle, a hum of power, the click of the e-unit, the smell of ozone...heaven ....The new stuff is a little quieter and smoother starting, runs slow better usually (Williams are still kinda fast)... know your style preference, including looping vs operations & reversing often. I eventually narrowed my fav. down to looping quietly at night with passenger cars; my indoor campfire. It started holiday evenings long long ago and trumped operations in long run. (best op. acc. gadget for kids is magnetic crane imo )
Fastrack has a different tone than old tube track. I find it louder, with a higher pitch... I.e. like "white noise”/static. I hate the sound, dont like the contacts, dont like the plasticy look. It does assemble ok, doesn't mark up wood/tile floors, and has lower chance of carpet fiber issues, lower chance of staining carpet with dirty grease/ oil if you don't maintain things religiously and immaculately. (Menards tube track... check it out for a bang for the buck, Lionel switches, and or go straight to GarGraves or Ross for wood ties, etc. (yep, even for a Christmas layout)
Your bell and chuff mean modern boards, but maybe not a can motor. Crew talk may not be avoidable; it's either present or not. Air whistle(no bell) vs speaker whistle w/bell, but both may have a board if newer.
PW bells are only in small steam switchers. New or old are great! Old can be pricey. The switchers were one of the earliest cast trains and scale too. Robust, detailed, but I'd claim that one and use it as a lure into kiddo learning responsibility. (Running better trains were a reward for adult-like behavior here...hundreds of them )
You can more or less lump any non-can motor onto my Post War suggestions. Whistle/Smoke? Some do, some don't. Pellet types can use liquid, don't run liquid ones dry. Post war pellet type smoke will more likely survive kiddo forgetting to fill. They can run dry for years/decades.
If you are mechanically inclined, with some electromechanical understanding too, the shallow end of modern electronics isn't going to be hard imo. You likely know help is right here already, and we aren't the only ones around; so fears of choosing a future un-mod-able doorstop are pretty minimal. (a can motor being discontinued might be a pain to hunt a swap for, but usually can be done. That swap would require more mechanical skill than electric)
That said, my first train was waiting for me to be born, Great Gramps and Gramps both Lionel men. I honestly couldn't care if had been used. In fact, someone elses old memories to tag mine to would make it even more desirable to old me (if that is possible )
Oh the memories: I was setting up track and running trains pre-school. Everyone learns differently, but in another 2-3 years.... no telling what kiddo will be up to. My first wasn't kept from me and there were many plastic locos later on too. The cast shell and bullet proof nature of Post War steamers are what survived me and lil' bro's abuse. I don't think the detail of the modern ones are as robust; shells vary, gears may be even be plastic in some new ones. (fine on some, but not all)
One thing to look at is if the loco can freewheel if pushed. You want that so kiddo doesn't strip gearing because; oh yes, there will be pushing too
Very small locos often use less voltage. Young skins moisture will feel pin pricks across track on command votage (heck, your foream will). Safe, but scary and does hurt a little 😂 Smaller transformers and voltages might be nice if kiddo scares easy.
Magnetraction rebuilding bushings/gears are special and getting hard to come by for some locos (2037). Rebuiding aa2037 can be done with unmodified 2026 parts, but the magnetraction might not be savable as a feature. Magnetraction stops rolling over in the curves more than anything really. Better traction too, but tires on new ones, just as good or better sometimes. (changing them a pain sometimes, consider how easy your choice is or isn't)
If being prototypical, an American style Adriatic 2-6-4 was never built for regular service so pick a Prarie, Hudson, Pacific, etc. (just maybe SF added a booster truck to a Prarie for tests) K-4 class lights are up high and not too hard to break off with a high speed wreck (oh yes, there will be speeding and wrecks... likely on purpose 😁... but look at the physics learned first hand 🤔) I didn't really like my larger 4 driver steam. I like the little 4 driver units though. I think it is the proportions and valve gear. The dual guide Walcheart types aren't visually appealing to me. They strike me as cheap (far from real world expectations, but true to my eye)
If you want pulling power, I suggest a large PW type Hudson like a 2046 (vs smaller semiscale like 665... both are fine, but the big Hudsons are beasts). Four digits on PW means 0-27 is ok for sure, 3 digit is a maybe on 0-27 but 0-31+ is fine. ..Berks have more overhang at the cab than up front, & are more involved to work on. (newer versions are often a whole different animal underneath. E.g. Getting in my new age Berk jr is kinda easy , 4 screws...plop and still runs with all gear moving. Berks pull great all sizes I've ran)
Overall, I think a 2 wheel pilot truck on 6 driver units are the way to go; easier to rail them and 4 wheel pilot trucks derail more often for me. ( except for the Hudsons. It's like magic; I can't explain why) 0-4-0, 0-4-2, better than ones with pilot wheels. I'd avoid shoe pickups and go for rollers mostly. Shoes are ok, but have more drag and more issues on turnouts, uncouple tracks, crosses, etc.
Weight is traction, but can also do more damage and be hard for the young to lift and handle. A cheap plastic 0-4-0 loco with can motor has some advantages for a young kid to learn with. It may not survive them, but it's cheap and learning that it sucks when toys break teaches you to care for other things better. (I had both )
A $5 bridge rectifier on a cheapie DC only stater loco from the 70s-80s runs on ac track and flies off the track at 8-9volt, runs with as low as 3-4v , and after adding weight, pulls like a big train. Also good for guilt free modification/paint with little fear of destruction because it is "valueless" and common.
Wikipedia: Whyte Notations, find the drawing chart; Looking at distance from the lead driver to pilot beam or cowcatcher. That will clue you to the overhang to expect up front on each loco type. The longer that is, the more overhang; simple pivot to arc geometry.
It may be easier to choose one or two styles and ask how versions may differ.
Below: One direction plastic dc cheapie..now ac too, cab forward, added Corgie plow, Lego flag, fun harlequin paint/weathered, cab headlight, etc. (reverses by manual switch actually)....junker repair under $25 total cost including new motor, BR (2" square (oversized too) 4 wires, 2 ac in & 2 dc out), & goodies... very quiet, repaired in an hour or so, tops. 3 hours including a drive to a train store and back... with a good look around too
My favorite Christmas train is The General/American 4-4-0 though. Very antique-y and the women in the family really like that Old Timer-ish look too Not always great pullers, the version quality varies greatly too. Usually quiet though, even PW....just plain cool imo.