Why dont you get a 30 gal instead. Rhoms grow 4-5" quickly within a yr but slows down after, growing an inch per yr after that. But for a 20 gal, start with a juvenile Rhom. A 2"er would be a good size for you to watch your Rhom to grow until he's about 5" and need to upgrade to a bigger tank.
An AC 200 would do nice for filteration (no powerhead, too strong for a 20 gal), 78-82 degree temp emmitted from a 100 watt Ebo-Jag heater, blue/soft lighting, PH of 6.5-7.0...
Anyting I left out??
You can also check out the link forum to see if theres a LFS close to you or other links where you can purchase equipments online.