@Kim Handelman,
There is some really solid advice on this thread. Here in CO, build cost of $150/sq ft means you're doing well. Get some quotes on asbestos mitigation. I know that disposal of that is on the high end, no matter if you or a developer does it. If there's a way to determine wetlands, you would be doing yourself a huge favor. That too, is usually a very particular process with strict adherence. Basically, anything environmental will be hard costs that you can't/shouldn't really side step. Is there another avenue you can explore with a/the developer you partner with, as far as financing goes? It will be a very long play, your money will be tied up for a significant amount of time & I think the cost of money alone would hurt any margin you've built in.
As a side note; how big is the lake? If your 5 houses already have services (water, sewer, elec, gas, fiber) you might see what the cost is to run those services to the new lots. It typically makes the lots or houses more attractive to would-be buyers, & making your buyer's market much larger. Not everyone knows how to care for a septic system, run on well water. You or the developer will probably have to run elec & fiber out there anyway. I'm not trying to say it's not a good deal, or you shouldn't do it. Just throwing some thoughts out there.