I seem to constantly have repairs for an 8 year old home (which the tenant does not even pay on time), including leaking faucets, fallen fences caused by overgrown trees falling, broken exhaust fans, etc.

Your deals have to withstand these costs.

So, I would exhaust HELOC first, before using my own funds.

I have definitely not run your numbers through the exhaustive spreadsheets that you have access to as a Pro member, but a really rough rule of thumb (I might get my hand slapped for this) that many investors I work with as a realtor in my area use is that you should gross monthly 1% of your purchase price.

Our alternative is a in/out exhaust fan at the top of the vault OR a dual zone minisplit (which is twice the price). 

I enjoyed my time there, got a good education, and made some life-long friends.I read through half the posts on here and just got exhausted.

The contractor's board will only interfere if you have made or exhausted all your efforts to resolve the issue, if you haven't tried to exhaust all efforts, you will simply get pissed because they will tell you to.

If that didn't work I would approach the owner.Again if I wasn't predisposed to one way over another I would probably do it in that order because it let me exhaust both options.

As of now, I'm just working backwards, figuring out from my agent what Sold prices per foot the area can withstand, designing a house with square footage to match the neighborhood, making sure there's a large enough margin between what my turnkey cost is and what I think it will sell for, and then finally splitting what the $ left between what I can offer for a lot and what I wanna keep for profit. 

If the purchase price is right today and rents offset the loan, repairs, and tenant turnover, then you can probably withstand a market correction and the loan will be that much closer to being paid off.