IMO do what oddballer said. Go with overflow to a sump, then one overflow standpipe down to almost the bottom. Almost everyone I know is using standpipe(which is just a overflow that goes almost the the bottom to the tank) to get the junk that sits on the bottom. At least on bare bottoms. Valves for water changes you can always figure in somewhere in the line or seperate on the lower side, etc.
If the stand pipe itself doesn't do a good enough job add clown loaches to stir everything up if stocklist permits. (That is a Li trick.. I know someone who's currently using it)
On the returns from the sump you can always have the pumps shower down I'm guessing..
Are you worried about getting junk off the bottom or you want easy water changes? or both?
There are probably numerous ways to set something like this up depending on what your priority is.