Upgrading a non-MTH engine is a bit more difficult than an older MTH engine. Most times mods have to be made to install the engine connector plug and tach reader/flywheel/tape, not impossible but something to think about.
I prefer PS2 in steam and ERR in diesel, reason being is the ERR kit needs something on the axle(s) to activate the chuff (at least that's what I was lead to believe), but that may be just as easy as getting the tach reader/tape/flywheel installed for the PS2.
Other than drilling holes, having enough space to install everything, and the plug on the engine, PS2 is pretty straight-forward. Downloading the files has not been a problem for me.
I've done 5 steam PS2 upgrades and 1 diesel PS2 upgrade. I've done 3 ERR upgrade. It's pretty cool when your engine comes alive after doing these upgrades