Ok so Ive been shopping and put the toilet and tank in position to check distances.
The design has changed so now the plan is to cut out the smaller floor section and replace this with a false floor made of hardwood for example. This will give me a design which leaves the flotation chamber intact and also makes it really easy to access the ball valves and diverter.
The total cost looks to be about $1300 plus the cost of fitting the skin fittings by the shipwright. Hopefully drilling a couple of holes wont cost more than a couple of hundred dollars, so I can stick to the $1500 original budget.
I don't have much choice about placing the tank above the water line. So mounting it high with some stainless brackets and using gravity to feed the overboard discharge makes sense to me. One less pump to worry about.
One question I still have is what hull fitting to use for the tank vent. Should I just use a 1 inch skin fitting or what???