Hey. Welcome to the forums.
I'd suggest staying away from a 55gallon, they are really too narrow to work well as a paludarium.
The 40Breeder is a pretty nice size, and sometimes you can get them at the petco $1/gallon sale, which is another plus.
When you do a paludarium, you can loose a lot of space in the tank just from the backdrop, and even more if you are building some sort of ledge type feature, so it's better if you can start out with a fairly wide/deep tank.
I'm sorta planning a similar setup, but haven't had much free time to get very far towards building it. I'm using a 40B, and want to have an extended frame/top which will provide some arboreal/ledge areas, while leaving most of the tank volume for water. I haven't settled on exactly how I'm going to build it yet, but I'm probably going to build it out of plywood, and give it a couple coats of epoxy.