the overflow box does not have to be $$$
provided you can find the right glue, you can get the parts and plastic to build just about sized overflow for less than $50... or if you want to really ghetto-rig something, you can use pre-fab plastic boxes and cheap plumbing parts to make a low-capacity overflow.
as for your pump, a wet/dry does not need an enormous flow, so any inexpensive pump (or powerhead) that can do a few 100
gph should work.