Maybe credit cards, a line of credit from the bank.

I do have a low interest home improvement credit card at 7% and I have up to 17k to use on that line of credit and about 5k saved.I'm afraid I'm going to max myself out... afraid I'll buy too low and too much work will be needed... or buy too high and not have enough to finish.

I have had temporary business cards made.

As an aside: Contact your credit card company and tell them what happened and most likely you'll get a 'charge back'.

These would have to look good and easily readable on things as small as business cards, and things as large as real estate signs/billboards.

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).  

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..

Also I will probably be utilizing some payment system for rent so my tenants could pay rent by cash/card/money order there.

If ANYTHING goes even slightly wrong, make them think it’s a HUGE deal, apologize, tell them you’re going going to have a long chat with the cleaners, offer gift cards to local restaurants, etc.