Don, you're a little late to the train... at least for locomotives.
The previous production RMT items were, outside of the ore car (which was the closest to scale of all the RMT rolling stock), were all traditionally-sized and thus looked great on a small layout. I personally liked the smaller sized cars, but as we all know, opinions and preferences are all over the place today in the hobby. The newer box cars are all larger, along the size of a WBB box car. The quad hoppers are the size of a postwar Lionel version.
But compared with prices on Lionel and MTH traditional rolling stock, the new RMT ones are still a deal. The Menards stuff might be less retail, but you have to consider shipping. And then there's the advantage of being at a show and actually seeing these RMT products. People don't consider that advantage until you have been shipped a defective product and have to pay return shipping out of your pocket.
Of the prior production RMT locomotives, the "BANG" S-4 and RDC "Buddy" are my favorites. Based upon the prior K-Line versions, the RMT models have many visual and operational improvements. Most notable are the higher voltage motors, so that these two locomotives can be run with a typical postwar transformer and NOT have the jack-rabbit starts. Yes, they have 3-position e-units, directional headlights, flashing strobe lights, plus the BANG S-4 has a horn. It's not Railsounds by any means, but certainly acceptable for a starter type locomotive. Pulling power of the BANG and BUDDY are far superior to that of the BEEP.
Your grandson might like the idea of the BUDDY RDC, as you could pull a passenger train or a freight train. Though there are no passenger silhouettes in the RDC BUDDY, you could certainly add them.
The couplers on the BANG and BUDDY are die cast instead of plastic as with the prior K-Line versions. BUT the coupler swing on the RMT versions is stiff, especially on 027 curves. I discovered you can push the coupler all the way to either left or right, and it will snap into place, staying all the way over to one side, which helps to break in the spring, thus improving operation on 027 curves.
The prices though on the BANG and the Buddy are not anywhere as appealing as they were during the time they were being produced. I paid below $100 for any of mine... not so easy now. You're likely to find better prices on the BEEP as they made so many of them, and the BEEF which obviously didn't do as well sales-wise. The BEEF was kind of a odd loco with the 027 length but more scale height and width, plus all the scale detailing. I would have been all over that loco had they stuck to the 027 concept as with the BEEP.
The other small thought to consider when buying any out of production engine is the ability to find replacement parts in the future. Makes sense to buy an extra engine these days for that purpose. At least with the BANG and the BUDDY, though there are differences, some of the previous K-Line parts can be used (or adapted to be used) on these two models. That would go for the GG1 also as that was also made by K-Line. Though the K-Line S-2 and RDC were made in far greater numbers than the GG1. Also the K-Line Alco FA and MP-15 also share some common parts. Just something to consider in this day and age where parts, even for current production items, are harder to find, never mind for products that are no longer being made.
Have fun at the show. If you see a BANG or a BUDDY for under $100, I'd grab it if I was you.