look for a chassis with support in your area. it make it easier to be able to run at the track knowing someone will have the parts to keep you running. As far as construction, i would look for a chassis that has a proven history working at masters weights. I would stay away from anything small diameter unless it has a proven history, especially if your just starting out. Small diameter(28mm, 30mm) will work, but it may take more work to get it working properly which if your new, may take more time then you have to figure it all out. I’d say that a 32mm would be what your looking for if your going to be running club races and just getting started just because it is a little easier to use with the heavier weights.
In terms of what class, again start with whats supported in your area. Don’t force yourself into a class of 1 if you can avoid it. If there is a Rotax masters class with decent entries and you would fit into it, then go ahead and run that. but if there is not one offered, then go ahead and find a weel supported class that way you can ask others for advice and information.