Welcome to The Planted Tank!
I would recommend that you purchase the biggest tank that you can. So, if you can find a 50 gallon locally, go with that. I'm not sure that the bottom of you tank being tempered is a problem. Many people look for tanks without a tempered bottom so that they can drill them for pumps or filters. If you are going to build the stand yourself take into account your height for optimal viewing, but be sure that you can reach the bottom for maintenance. That size is definitely not to shallow or deep. Just try 'scaping a 29 gallon