easiest would be to buy a large canister or 2 and plug and play. Cheapest would be to build a sump filter using an old tank. You can get old tanks that are beat up and or scratched to where they are no good for show and use them. That way the only real cost would be in a pump. everything else needed can be made from free stuff like old windows for the glass baffles. Of course media would be another cost but that is in etheir situation.
DIY sump would be cheepest assuming the tank is drilled allready. Easiest would be two big cansiters (FX5) and on ebay you should be able to get 2 FX5s shipped for right around $360 then use the other $40 to buy mesh bags and bio media and there you go.
Maybe add a good powerhead (koralia) to get extra movement.
A sump is best. Don't need extra AC110s for movement. Thats why you buy a good powerful pump and branch off to multiple outputs with small diameter tubing to keep velocity up.