I'm worried that it would be difficult to find a good selection of tenants in such small town.

The houses are 70 to 100 years old and they seem to be fairly small, ranging from about 750-1200 square feet. 

I believe most AC beyond 18,000 BTU are 220 volts, but some years back, 110 volt units can still be found.I don't know how cool your AC's keep the units, but when the AC is borderline, it's virtually useless as tenants keep complaining there's something wrong with the unit, or it's too small.

.#5 11unit single family compound (9 small 2/1 and 1/1s, two of which are duplexes) with a 10 unit storage building and 1 extra lot for a possible RV rental.

Assuming, as you said, that most of the profits are going to the downpayment of the next house, then it seems you either made a small profit or you are moving to a big house!

I currently work in Market Research, and am interested in investing in single family/small multi-family (2-4 unit) rentals in the area (close enough that I can hopefully manage them myself).

.- Do Small banks (in STL) offer out of state investors loans?

I grew up in a small agriculture community close to Lodi and have always been interested in owning real estate but did not really know where to start. 

Doubtful, but it's in the lease if I ever wanted to take them to small claims court.

I run a small business currently but I’m looking to get into the real estate investing game.