Take a water hose on the roof and try and flush out the vent.

On one side there was a small leak in the blue poly water main near the meter and on the other side when you flushed the toilet water went in to the bath tub. 

Every month I go into a rental for "scheduled maintenance" to change AC filters, bleach out AC condensate lines, flush water heater T&P valves etc...one thing I do is to exercise open/close of all stop valves. 

My opinion is that you’d be flushing your money down your shiny new bathroom’s toilet to add that 2nd full bath.  

Unless they are flush with cash they are desperate to make a nut.

You may have to pay a few hundred dollars for each inspection, but it will be well worth it to know if they are in good condition or need repair/replacement, then you can go back to the seller and renegotiate.As far as owning goes, just make sure you and your tenant treat the septic correctly and not just flush anything, since you'll be on site that should be easier to control then if you were absentee landlord.  

You can do that and then wait to bill back to tenants until their leases come up for renewal, but then you risk them not agreeing and moving out and now you have a vacancy.I would replace any old 1.6 gallon per flush toilets with newer 1.2s.

I've heard varying recommendations in terms of how often you should get them flushed to prevent hard water buildup.

I would say given your location, its going to be tough to fund projects with cash even if you're flush with it.

All major moving parts and appliances in the home seem to be in working order water runs toilets flush theirs heat but no central A/C, fuse boxes with mostly 2 prong outlets.