Anytime you have an extra heavy dirt load on a pool, a bottom drain makes maintenance easier, and improves circulation. In your situation, I think those factors will be pretty important.

The alkalinity won't be a problem, if you have a waterfall. If you maintain a pH of 7.2-7.4, the waterfall with remove most of the alkalinity (via aeration) over time. But, you'll probably have to add a lot of acid.

I'd recommend that you consider an oversize DE filter, like the large Sta-Rite models. Since you may need to keep chemical levels down, the extra dirt removal capability of a DE filter (compared to a sand filter) may well be significant.

Ben
"PoolDoc"