In some cases people paid down mortgages ahead of amortization, in others they may have done interest only loans, or negative amortization loans, so there are some wild cards to the calculations. 

I have already used 0% 15 month APR credit cards and cash for work on the house remodel (yes, I have a plan to pay them off).I was thinking of getting a HELOC for the 60k loan, but my DTI is too high ( and my parents have no income for co-signing).  

There has to be some form of a written agreement on who gets what of the profit, who's in charge of maintenance, what happens when a partner wants out, who's on the loan/title... etc  I find it hard to believe that this person wouldn't just take this deal and run with it on their own since they are funding everything with their money and I won't be giving a dime.

I charge my tenants an $80 flat fee for water and trash. 

seller knows more than you... and unless you have the cash to buy this  closing in 14 days is probably not in the cards unless your getting a smoking deal which from the sounds of the sellers sophistication is doubtful..

That's considered "neglect" or "abuse" and I would seriously consider charging the tenant for at least a portion of the repair.In the future, you should only renew after a complete review of the tenant's payment history, communication, care of the home, etc.

Also there is a laundry room that he currently doesn’t charge for, I’ll be adding coin op laundry and charging a small amount. 

My lender is charging me $2400 for the rate lock.

He is charging me $2400 for the rate lock.

If the RE properties were in your name(Not in LLC) and it was rented directly to medical practice as your capacity as an owner, there would be Self - Charge interest rules, but I am guessing the ownership of the RE properties is under LLC whose business activity is rental, so those rules can be bypassed if structured carefully.