You can work with a local credit union that does HELOCs, Home Equity Lines of Credit or Loans, on investment property.

Sounds like the person knew what was going on with their credit report as they have filed reports in the past. 

Seller financed (probably) won't be reported to the credit bureaus and so won't impact your credit score at all.

Platforms like Zillow or Redfin can give you insight into comparable properties, and PropStream is great for deeper analysis like liens or ownership history. 

Credit pulls for the same type of loan within a short window (usually 14-45 days) are treated as one inquiry, so your score should be fine.

Just as a comparison for using greater Cincinnati credit union they have at top tier credit score 6-7% on 7-12 year fixed rate loans, 9.5 on variable loans and 5.25-6% on 10-30 year loans.

Alternatively, you may want to consider switching to a platform that offers this level of transparency.Additionally, on both Airbnb and Vrbo, if you have access to the account, you can view the transaction history.

Also, would this loan help build my personal credit, or would it only report to business credit bureaus under my LLC?

Assuming you go the HELOC route, another option to reduce your costs assuming you have decent credit is to look at credit card balance transfer promotions.Back in 2003-2004 I had HELOCs on my investment properties and interest rates were increasing.

Verifying last 2 years of rental history very important!