You need to define FMV and the procedure to obtain it, there will be disagreements on the calculation, time periods, cost allocations, closing date, etc.

Things to consider: Purchased the bigger lot for $600k under contract for deed with 4% interest for a five year period with $30k downPurchased the small lot for $210k with 3.5% interest with $172k left to pay.

It would be difficult, rare, and frankly not a practice I would help with, for you to place your downleg under contract and your upleg under contract before your downleg is past due diligence period and the buyer's money is hard.

You can often find lenders with no or low "seasoning period," which means you can refinance after 0-6 months after purchasing the property. 

It works out because we have been studying since October 2021 yet there is still so much to learn and that can be done during this period. 

I also let them leave at any time during that 60-day period with just a week's notice so they have more control and know I'm not going to screw them over in the process.

If so many lenders will have a 6 month seasoning period before you can take out a loan.

You would need someone to fill in for you who is capable of handling anything thrown their when you are unable to be reached for a long period of time. 

If so how long should this exclusivity period be for?

I worked out the numbers for both and the HELOC would save you about $70K assuming that it is a fixed rate of 4.625 with a 10 year draw and 20 year repayment period.