I am not a fan of the Wealth system either, but the True 20 Freeport Companion has a nice hard currency system and an abundance of handy True20 expansion.
To keep the game simple and easy to run, I would keep the books down to: the Core rules, the True 20 Companion, the True 20 Bestiary, and the True 20 Freeport Companion (even if you don't use Freeport, there is a lot of information there). If you are running fantasy, Fantasy Paths would work well as a handy reference. A copy or two of the True 20 Pocket Player's Guide for your players would also be useful and there are a few quick reference sheets out there to help the game run smoothly.
Worlds of Adventure is a nice product as well, maybe not necessary, but every True 20 book is handy, Green Ronin doesn't put out useless books.
edit: I forgot True Sorcery, which I use as an expanded magic system to the one in the core book. It is not necessary for play and is actually a d20 book with a section on using the system with True 20.