What if we charge a consultant or marketing instead of finders fee?

When we last spoke he mentioned he had covid and his phone has been shut off, but he was planning to be at the house the next weekend with rent and all the late fees ready to go.

Even late fees.

Yes, most of the reputable lenders have a min loan amount of 65k - 75k with a minimum of 40k - 50k of that amount going toward the purchase.I've been down this road before with a few investors who want to buy cheaper homes and what I can tell you is that - 1) It's not easy and 2) You WILL NOT like the rates, fees, and points.Think about it, what does a lender stand to make on an interest only loan for about 40k. 

There are AC service company offer a reasonable fee for a annual plan where they come to change the filter twice a year.

Taking the market rents, accounting for a 10% vacancy, the debt service, PM fees and everything else, it seems like I should be able to cash flow about $600 for the 4 doors combined.

Find out what the going rate is for rent, increase it a bit for the shorter term and if they stay over 14 days, have a per day fee that escalates if they stay longer.They can always get short term rental to stay in and put their stuff in storage.I would NOT start any work until they are out. 

As to assignment fee, the average is said to be $10,000, with actual fee ranging from $2,000 to $50,000 or more.

And is cleaning fee part of revenue?

You might be able to find a contractor that will agree to be your qualifier for a fee, etc.