Use schedule C PVC plumbing and hardpipe all your sinks/faucets. 

I'd have to sink at least $10k worth of work in addition to the $18,000 down payment but I'd get the 10k back on the option consideration. 

If its LR1, LR2, LR3, NC, or any other high density zoning I wouldn't sink any money into them - highest and best use is to tear them down and rebuild in those zones.

That shows buyer demand in the marketplace.You could go out and list like gangbusters but if the product you are selling is not desirable in the marketplace then you are on a sinking ship.Have inventory of what buyers want to buy and not want sellers want to sell. 

I'll let that sink in and figure out what you want to do. 

Ultimately, my question is how much a running toilet / leaking kitchen sink could reasonably impact average water bills.

Then they just have to install and cut a hole for the sink and faucet.....

If I update the kitchen (new cabinets, counters, flooring, sink , etc that is probably 15K-20K if you put some sweat equity in you can probably get it down to $10K-12K.

LOL, you'd probably win the weasel award.And I keepsaying this, but it's not sinking in, a seller financed note made from equity can not seek any deficiency amount.

Deep clean the whole place, pull the fridge out and vacuum the coils, make sure all drains are working and sanitize them - stop and fill each sink with a mix of hot water, a splash of bleach, and several good squirts of Dawn dish soap.