You should look at getting a min sump size of 20% of your main tank size. For you that would be 15 gal
. You can go the cheaper way and go to walmart and get a plastic tote and run the water through that. One thing you might want to consider is drilling your tank for the outflow and return flow. I wish i had because i have to run an overflow box and they worry me(flooding). Basically you need pipe to get the water to the sump, a return water pump and pipe to run it back to the main tank.
65 gal reef
2 PC's 192W each , 20 gal sump/fuge
4 false clowns, 2 firefish, cardinal, midas blennie, toadstool, zoos, shrooms, trumpet coral, torch, pagoda, frogspawn, poylps, red open brain, devils hand, hermits, snails, 3 cleaner shrimp, coral banded and a emerald crab.