I've seen it used many times since though...or variations of it

If you have a lot of variation, you need a thinner plank (or even a glue-down) to follow the floor's contour or you need to level the floor really well.

I don't have a preference for thickness one way or the other and instead tailor that towards the floor I am working on - if the floor has a lot of variation in it, you want a thinner plank that can follow the contour.

There are some variations depending where you live.

Sometimes its trial and error.Lastly, make SURE, you tie your payment schedule to significant deliverables to make sure the value matches your cash outlay

So far there’s a couple different softwares I’m going to do a trial on to compare as I’d like it to be simple and functional, but also provide accounting data.

There is always some trial and error. 

If it is zoned as a single family you will need to obtain a variation to use as something else.

There a way to go to court and force the other party to sell, its called "Partition Suit"  however, that's going to take filing a lawsuit, going to trial and paying an attorney. 

I suggest whatever calculator you use, do some trial runs with actual properties.