Many times they will claim it's wonderful and then try to hide their worst deals by only showing completed deals.

What is the experience of the group at performing this sort of development?  

But worst case they should assume you’ve paid it off, helping you. 

Just trying to prepare for the worst.

The fact that you have made such good time on the rehab of this one means that you will not be able to refinance on its new value until you have owned it for 12 months, otherwise they will go off of the 'value' that you bought it at.Whereas if you refinanced it into a 30 year DSCR loan, you would likely be good right now, or worst case most are good at about 90 days.

The worst lenders I have dealt with are the ones with underwriters in one time zone and origination department in another, branch manager in another that don’t communicate well with each other and end up over-promising while not being able to deliver.

Make sure the numbers so that if you put in 60k to 70k you're getting some sort of annual net profit so you aren't having to feed the investment. 

What else should I be working on in the meantime prior to listing the lots for sale to get ready for this sort of transaction?

If your "estimate" works for you and your "high estimate" is still workable for you, then you can feel confident that if you use good project management practices that you can manage the project to somewhere between those 2 numbers at worst.

Again, you certainly don't have to verify it this way and to some extent that is why most invest with turnkey companies - it makes it easy and is a sort of one-stop-shop but I think your definition of doing your 'homework' may be different than my definition.