Before anyone can go on about equipment help, we're gonna need the size of the tank your planning to house your Rhom in. Reason for this is so we can determine the right amount of heater wattage, filteration..etc. Also the size of the Rhom your planning to get in have it happily housed in your tank.
For purchase of your Rhom, best places are:
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.