Go to BUILD YOUR TEAM at the top of the screen and look for local investors or meetups in your area.

I just had to pay 11k for a new Hvac system on one of my buildings.

They said we have to build new walls because the old walls are directly on the concrete, and this is not allowed.

Compounding on one another sure that will have a big impact on the market but there are a lot of factors at play.

The seller has the two lots legally separated, but due to deed restriction imposed by the subdivision development, found that building a house on the “tennis court” lot was not allowable. 

I do STRs, building some now. 

In my market, which is a fairly economical place to build, I am friends with full time builders that are in their properties about $150k/door for construction level finishes.  

A reserve is unnecessary, but I still keep around $15,000 - $20,000 in my account.The point is, that you should sit down and assess your finances to determine what the worst-case scenario may look like, how much you would need to cover it without impacting your life, and whether you will need to build a reserve.

The options are, demo the lot, split the lot and then sell 2 lots to the builder or build myself.

Chris answer is spot on.I'll add "quick closing".You won't find investors easily, you have to go to a lot of meetups to build them.