Advice Please.

I need advice on two things.
First is a horse that I am training called Lexy. She is a very nice little 2 yr old mare that I am training for western pleasure, trail and showmanship. Eventually she will be for a 12 yr old girl (next summer sometime). Today during our showmanship lesson she was doing very well but will not stand with both back legs on the ground. She is constantly resting one. So I started working on that but then noticed that sometimes both feet ARE actually flat on the ground but her fetlocks buckle forward. only one at a time and she can stand with it correctly but prefers to have it buckled forward. I will work on getting some pictures of it today. It looks almost like she is resting her foot but her hoof is completely flat on the ground. When she does put it back in place it looks like it pops back into place. Very strange.
Second- I have a building that is about 30'x50' (maybe a little bigger) and originally I was going to put some stalls in it and use it as hay storage. However I am getting more and more frustrated with not being able to do anything when it rains and then not being able to do much for a few days after while things dry out. I was thinking today that maybe I would just turn it into a small (very small) indoor arena for a while until I can actually afford to build a bigger barn with a big attached indoor. I am figuring that I wouldn't be able to do much, if any, canter work in there but I could lunge, do walk/trot, showmanship and other ground work. It would allow me to at least do something in the winter as well as right now the horses get from December to February off since the ground is so frozen and everything. Also it would mean that I don't have to cancel lessons for 2 days every time it rains. My outdoor is dirt and gets slick really fast. What do you guys think? Is that way to small to even consider putting the work into it? It already has a roof, electricity, and 1 side with big sliding double doors. So I would just need to add 3 sides, lights, footing (it is just dirt right now), and a 1/2 wall all the way around. Then if I ever decided to turn it into a barn I would just have to pull out the footing and put in stalls. I just don't know if it would be worth it for that small of space.