I don't want to start a back and forth, but don't think a 40gal would suffice a 240gal display. Our sump is 90gal with an additional 30gal fuge. I would try and get the maximum size that fits under your stand (if this is where you want it). What are the dimensions of you stand? If you're looking to build one, do a search on here for sumps/fuge and I'm sure you can find a plan. Also look at Reef Central, there are probably 5 people a week building their own sumps on there.

If you want to buy, go by Gabe's and he can hook you with with one of his sump/fuge combos.