Then locking the dog in the 1/2 bath it proceeded to tear out the toilet water line.

Seller knew it or not.Call a plumber and ask to have it capped professionally.Text them pic first and ask for a bid.Tell them you want to concrete over the cap--forever.ask them if they can/will (cost) to concrete over it.In some areas they carry concrete in their trucks to set new toilets on (other place forbid it).Good Luck!

For example, if you don't want to be woken up with calls in the middle of the night about clogged toilets, say so in the lease! 

That is a good first step, at least you know what your numbers are and can offer on others if this one doesn't pan out.

A co-signer is fine, and I've had those pan out OK; generally not an ideal sign either, but OK.

Do not buy $40K-$60K properties Anything in this price range is going to be in very rough neighborhoods and although they look good on paper, they do not pan out unless you are local, very hands on and have a lot of experience with this class of property.

When we have 4 bed houses, 60's or newer, or previously renovated houses, is where we start seeing those much wanted 2nd toilets lol.

If its true and if it panned out with due diligence.

The basement is finished and has a toilet and sink, but no shower or kitchen.

You would need to submit planning for this, get approved, and get two inspections for this from the county-wide health department.If the bathroom for this ADU was already in the basement (I assume it was, or you would have mentioned it, as the 4-inch branch drain for the toilet would have been far more difficult and expensive than the minimum 1 1/2 inch brain drain you need for a kitchen), you would have to worry about upgrading the egress practicalities as soon as that drain went in.