HVAC is old but functions - ask for seller to get them professionally serviced if it is not done yearly. 

Should I combine the marketing of my PM company and my buy/sell RE services in one?

•The estate itself is not necessarily a related party (since it’s a separate legal entity).

And a deal in my opinion is one that will 1. pay for itself(All property expenses) and 2. pay the debt service(if necessary) and 3. put money into my pocket each month.  

If not, don’t even think of this.No matter what, consult a qualified lending attorney to understand your legal options and ensure you have proper loan documents in place to protect your investment.

(As I navigate my service business, I've realized that there are more lucrative avenues available.

Check his legal history.

FYI - not legal advice.If it's not a formal assumption and it's a straight sub-to, typically the seller remain personally liable for the debt if the investor defaults.Essentially, what can the investor do better than the seller after refinancing the property? 

For instance, and in brief only using one concept (not meant to be the full list of criteria), a rural property has a population restrictions and proximity to Metropolitan Service Areas ("MSA" - think big city area) as some restrictions.

I've seen other people in the forum who simply use independent savings accounts but it is not clear to me if that is legal.