After some research online, I am reading that the cost per square foot on a gut renovation of this nature in Stamford CT is anywhere from $150-$350, which is ultimately not very helpful, because at $150 per sq ft, this is a very appealing project with a total profit of ~$175K (46% ROI) and at $350 per sq ft, it very much isn't, because it would be a net loss of 83K (haha).QUESTIONS:What can I do refine my estimate a bit given that I am not allowed to a formal inspection on the property? 

Sounds like you’ve taken the right steps so far by giving formal notice to vacate.

Answer sure.. have to check if there is a due on sale clause or if you have a PG to the orignal seller.. that mortgage stays in your name unless there is a formal assumption  Maybe the example is more challenging for a house as the property.Ex. with vacant land - seller finances to me $20k with 5% down and $1k/moCan I after 1-2 months, then resell the land to a cash buyer for $25k, pay off the original $20k and pocket the $5k?

In the meantime I would love to have some helpful information or ideas to share with the buyers when we meet and ideally an agreement/contract we could fill out together once new terms are agreed to and then share that with the attorney so he could draft up the formal instrument for recording.If you have experience with this type of scenario, I’d love to hear any recommendations you have or any helpful resources you could point me to for the appropriate paperwork.Under foreclosure laws, you have to send appropriate notices and try to work with the borrower and offer a loan mod before foreclosure anyway. 

For background, I'm partnered with an architect and interior designer, but I'd have to find most of the subs as I don't have any formal connections (at least in the area I'd like to build).

To establish a 50-50 ownership in both the property and future profits, you'll need a formal agreement outlining each person's contributions and rights.

Use your HELOC to purchase the duplex outright for $118K.Your brother’s $50K down payment can immediately go toward reducing your HELOC balance, leaving a remaining balance of $68K.Create a formal lease-to-own agreement with your brother.

@Matthew Cook I would NOT pay for the renovations unless I was on title or had a very formal contract written about the split of profits.

It’s a lot less formal than the residential side.

Although Baselane doesn't have a formal app I find it very mobile friendly, I use it on both my iPhone and IPadI think Baselane has a slight edge over Stessa so I think I'll start with them and switch to Stessa if it's not working for me.