This is all new to me so I’m sure I may have made some mistakes.

Biggest thing is understanding all the terms of the HELOC- when/if the interest rate is fixed or variable, what is the 'draw' period vs the period that you need to repay. 

that biggest fear I would have in this situation is when it will turn over.

According to Wyoming Public Media, 75% of land across the U.S.’s biggest cities is earmarked for SFHs, leaving other housing classes vying for a small piece of the land bank.

Anything to look out for along the way or mistakes that could of been avoided?

The Consumer Price Index soared 0.8% in April to match the biggest monthly increase since 2009, the Commerce Department reports.

Some mistakes, and learning from them can be costly.

To determine if it's worth doing a 4-Plex in addition to the cost you listed, I would recommend looking at; is the lot already graded, and ready for construction, are sewer, water, and gas already at the lot for connection, what type of rents are on your proforma, the biggest one many people forget about are the developer fees charged by the municipality to allow you to build the project, etc.There are a few soft cost items that will be a part of the project that do not get lower just because you're doing a smaller project.

They are also selling their current place, so will have a decent chunk of change once that closes.The high cost around closing are my biggest concern.

I told his realtor (his realtor is representing both of us, i know big mistake) he failed to disclose that within the time contingency written on contract but hes saying "that would be hard to prove".I really don't mind walking away, but I've already paid for inspection and appraisal plus travel costs, time taken off work, etc.