One recommendation might be to start with a diecast model first, they do present fewer challenges as a rule. I find that the MTH steam is usually the easiest models to upgrade. Before the craziness of MTH pricing, I used to look for older PS/1 Premier steam for upgrades, however now with MTH pricing through the roof it's a little less attractive.
One thing you'll find with older brass is the quality of the soldering can be spotty, and it could have deteriorated over the years as well. It's pretty distressing when pieces start falling off your prize brass locomotive.