While the G250 is a better 2nd stage than the R380, there is absolutely no reason to run out and buy your wife one even before she's certified. She'll already be using a regulator that's better than most OW students use, and one that could easily go to far beyond anything close to recreational depths.
2nd stages are constant volume devices, meaning they pass the same volume of air regardless of depth. The R380 will function about the same at 150ft as it will on the surface. It's the 1st stage that has to work harder to maintain a higher IP as depth increases.
The MK20 may or may not benefit from being upgraded with the MK25's composite piston, depending on which piston is currently in it. There are three generations of MK20 pistons; the middle brass tipped one is terrible. So if yours has one of those, it's worth immediately replacing. The only way to tell for sure is to take the regulator apart. But, a savvy repair tech should be able to get a good idea just from putting an IP gauge on it and watching how responsive the needle is.
As far as which G250, just avoid the G250HP as triggerman says and you'll be fine. Good luck finding an original G250 for $100 though! I think I paid about that for one at least 5 years ago, not in great shape and needing immediate service.
