We have had 3 people apply for the unit. 1 had horrible credit so was rejected. 1 was nice but disappeared. 1 we offered to and they chose a different place for location. 

If you are married and your wife has a better credit score, have them quote it in her name.  

I was advised just as you advised me James to either open up a credit card for the necessary amount to get the down payment, seller financing, private lender or sell my rental house and use that equity to buy the next.

I'm 33, no debt, and my credit score is 750+.

Shoulder buyers are those that are close to qualifying but come up a little short on down payment or credit.

That’s very different than if you were approved for a line of credit for $150k.  

I have doubts about taking on a mortgage at my age and whether it will affect my credit score positively or negatively.

I have good income & excellent credit as well.

On Commercial notes from community banks and credit unions you may notice that the loans will balloon at the end of the term period and often times the term is shorter than the period they amortize for your monthly payment so at the end of the term you're left with a sizeable balance to payoff or refinance again.

Most states will give credit for tax paid in other states but if their tax would have been higher they then assess the difference.