Foreign National programs sometimes require that you have US credit and most will want you to have a US-based bank account.

I am looking for recommendations for best financing options.The owner is considering financing it, but until they agree, I am planning as if they will not do so.The local credit union I use, who is very investor friendly, is unable to lend on properties larger than 4 units right now. 

Best bet is to try local and credit unions, possibly may have some programs for better rates.

Most of our newer investors approach 2-3 lenders to discuss loan options.Some lenders recommend making your financing inquiries within a 45-day time period to avoid larger drops in your credit scores.All the best!

Is there a way I can "co-sign" with one of my parent's credit cards and be on their credit history in order to boost my credit score? 

A couple issues with Credit/income on paper.

We have excellent credit and cash capital.

My budget is in the $200-250K range.Please feel free to let me know if additional details would help, and thank you in advance for your time and advice! 

In addition I am giving him freedom to explore (to a certain extent) on my dime.

Take stock of your current credit score and accounts.