Some of the clown hosting anemones can get really large. I once had a BTA fill about 2/3 of a 90 gal tank. Now I'm not saying you need a 90 gal tank. but it does give you some idea of what can be involved.
Yes, keeping anemones out of the filtration system usually involves multiple overflows each getting a modest flow of water. You need to be very careful about keeping them out of powerhead also. Heating is best done in the sump.
Anemones require high quality lighting and water and well established tanks. Be prepared to wait about a year after the tank is setup before you add an anemone.
Consider that you might get a much better looking tank, a lot sooner and with a lot less problems by going with something else. I know clown and an anemone are a favorite, but using other corals and things like zoas and mushrooms are a lot easier to keep.