MarshallE wrote:
My boat has 800 sq. ft. of wind surface and estimates are coming in to use
anywhere from 450 to 750 pounds of thrust.

I would tend toward a smaller unit and work at placing it as far below
the waterline as possible. Given two units of equal depth to the low
point of the tunnel, the bigger unit will be more likley to suck air and
lose effectiveness both because it is closer to the waterline and
because it's pulling harder.

Depth to the keel of the forefoot is a solid limit.

What brands do folks like and what should I expect to spend for a turn key
job?

Vetus.
$12,000 and up, not counting hauling & storing.
Some friends of ours just had one put in their 41' trawler.

Fair Skies
Doug King