When the units are empty change toilet flappers (and, if need be, toilet "guts"), check toilet bolts on tanks, the floor and the toilet seat. 

You are running a risk of the cesspool failing, and having to pay environmental costs and fees, and pay to replace the cesspool with a septic tank, so looking at potentially a $20k cost.

Sold the houseDeal details:3 BR.1bath brick house with attached 2 garage door in Flint, MIPurchase price including closing cost, inspection, etc: $20,673Rehab cost: $7,150 that I funded myself.I estimated $5,000 but went over because hot water tank and furnace were STOLEN while the property was listed for sale (any tips on how to avoid that other than avoiding such locations are appreciated)Rehab scope: minor plumbing, painting most bedrooms, installing light fixtures, replacing the carpet in one room, installing fence gate, landscaping, and other miscellaneous tasks.Sale cost: $6,250 including realtor fees, seller credit (3% of sale price), title insurance policy, etc.Profit: Sale price – [ acquisition price+ rehab cost+ sale cost]=49.000- [20,673+7,150+6,250] = $14,927Please feel free to comment on the following.Things that I should have handled differently?

Also consider the cost to install septic tank and water well.

Poor woman (tenant of 7 years) called me at 2am because her basement HW tank was floating sideways & banging against the floor joists.

Once they smell blood they are sharks in the water. 

Not sure on hot water tanks and furnace but I planned on finding out Tuesday.

You won't be able to tell the following:1) Does the hot water tank actually heat anything up.

Would have been terrible if i had to pay for clean up down the line or removal of a tank.

I am investigating purchasing another long term buy and hold property that has a septic tank.