Hi! I was in your shoes a few months ago. You’re going to be just fine.
Schoolmasters are wonderful, and will keep you incredibly honest. A horse who knows its changes and pirouettes will teach you to stay balanced in the canter…by popping changes when you lean sideways and turning a ten meter circle into a pirouette when you lean forward!
That said, there’s a world of difference between the kind of schoolmaster who says, “your foot was 1/8” out of position when you asked for the canter, so I’m not going," and the kind who says “I think I know what you’re asking, so I’m going to try it, even if I’m not sure.” Needless to say, you want the latter. There will be stuff you can’t necessarily get this horse to do, but make sure that when you’re trying it, you can get it to do all the things you currently know well, and a few things that you’re trying and experimenting with (i.e., I was doing first level work when I went looking and had taken a few lessons on more experienced horses where we worked on some second/third level movements, so my trainer had me make sure that on anything we were looking at, I could do a shallow serpentine and shoulder-in, haunches-in, half walk pirouettes, one flying change in each direction, etc.). Does the horse feel like he wants to try for you? Then he’s a solid candidate. Does he feel like he’s going to put up a fight all the time if you don’t ask exactly right? Those horses are great teachers for some people, but ask yourself really honestly if it’s going to drive you crazy if it takes you many months to get a solid walk/canter transition.
All the normal rules about looking for a good amateur’s horse apply to looking for a schoolmaster – is it kind? Is its instinct in confusion or panic to slow down or to speed up? Better that the brakes work a little too well than that the brakes be hard to find. The training needs to be correct, but I promise it really is okay if it trots for a 6.5 if it has a temperament for a 12.
If you want to show, ask what the horse is like at shows, and at the kind of shows you’ll go to. We took our new guy to a show a week and a half ago and it was blowing 40 mph and my coach was like deathly ill, and the fact that he was a solid citizen mattered tremendously. Ask all the questions.
It felt like a big scary commitment to buy an older horse (and in this case, one who had belonged to a beloved friend), but I swear, this is the best investment in my riding I have ever, ever made. I have made more progress in the 3.5 months he’s been with us than I had in years. It is so, so worth it.
Good luck! If he’s the right one, he’ll make you feel like you want to try stuff when you get on. If not, pass.