I have a two-and-a-half-year-old Half-Welsh, and he is wonderful. His other cross is a Paint; so I don't know how much he would compare to your prospect, but he is super calm and brave. Nothing bothers him, and he takes everything in stride. I don't know if his great personality is from the Welsh side, but he is very friendly and curious. You can't go out into the pasture without having him investigate what you're doing. He just started wearing a saddle, and he acts as if he has been doing it for years.
I wouldn't expect a lot of growth after two, maybe a few inches, of course it depends on the individual. I'm hoping mine won't grow any taller because he is already over 14 hands, but he has been filling out more than growing taller since he turned 2.