my 2 cents...
I owned a RS 750XXl and now a RS650P i fell the stands are lacking. but there CS has been pretty good so far they sent me a center brace and and a new set of hinges.
I believe WB stands are real plywood? i also had good experience with WB CS too.
that said reading what you want to do with custom stuff I would go with
@joe@glasscages.
get the tank you want. get a sump and build your own stand. will it be more work yes might it cost more probably but you get what you want.
also there are 2 ways to convert the RS plumbing to US standard. the sumps I find are pretty nice too. all depends on what you want for a sump.
and lastly check out fiji cube tanks and sumps might be what you are looking for?
good luck!