Learn anything unexpected?

Things your contractor should know: there are no 2x4s in the place, the switches and outlets don't have standard boxes behind them, the subfloor in the bathrooms almost always needs to be redone, the cabinets can't hold granite unless you built a support structure, support under the house where there are tubs, and the roofs cannot have more than one layer of shingles. 

Using OPM is going to be the next real way to grow, so I need to figure it out. Let

If there wasn't, he needed to provide notice as I'm sure the lease would have a clause about unexpected changes like income, friends moving in, etc.

Are there unexpected fees, red tape, things to be aware of when working with Hud housing that most people overlook?

When your roof leaks, the dishwasher breaks down, or the outlet stops working, you don't have too worry about it, the land lord does, and if you have a GOOD landlord, the landlord will take care of it quickly, but a lot of those kind of land lords are expensive.

In the basement, I have the option of plugging into either unit 1, 2 or 3's outlets.  

Wellllll... we received an unexpected visit from a city building inspector while we had a painter at work.

You need money for reserves, unexpected problems, occasional vacancies, and the like. 

Some of the most discussed turnkey providers on these forums would be REI Nation and Rent to Retirement so definitely check those out. Let