Seller says he won't go down at all or credit.

If they can’t or won’t answer those questions and are unwilling to give you a loan estimate without pulling a credit check, i walk away.

What if these are the lowest rates right now that we will have in the next decade? 

Can you pull a line of credit on one of your properties to use as an emergency fund? 

@Erwin Groenendijk, you are correct but Americans should be able to credit their taxes paid in Spain against their equivalent US taxes courtesy of the treaty for the avoidance of double taxation between the two countries.

Now, I can negotiate seller credits to address some of the plumbing issues and have them run a water line for the 2nd unit.

You can access equity of current primary by a Home Equity Line of Credit (HELOC) or refinance.

A lot of people think 20% is the standard only with certain lenders and Credit Unions. 

In your situation, you are getting a tax savings, which could theoretically be added to cashflow as a negative expense.As a flipper, I would be very concerned about the credit markets. 

They had a low credit score but beyond that, they were a solid applicant based on their application and references.