If they started rebuilding and sealing up walls, you need to approach it like any other property: assume you will need to redo plumbing and electrical. 

Catching it before plumbing is involved (if concrete) can save a lot of money on tunneling.

(Its on septic, check electric, plumb, framing, foundation is poured concrete.)

We need to re do a portion of the extension, new electric, new plumbing, new HVAC, new walls, new bathrooms, new flooring, new insulation new drywall, new kitchen, new doors and trim throughout the house.

This 4 sided brick ranch has all the upgrades you want including all new roof, new HVAC, new electrical, new plumbing & refinished original hardwood floors.

This 4 sided brick ranch has all the upgrades you want including all new roof, new HVAC, new electrical, new plumbing & refinished original hardwood floors.

They don't know whether the subject property or comps have knob & tube wiring, popcorn ceilings, cedar post supports, ancient plumbing, rust inside the electrical panel, floors that are out of level, foundation defects, termite damage, a failing septic system, well water that smells like rotten eggs or any of a thousand things that could drastically change the home's value and most of which are impossible to pick up from listing photos or tax records.They also don't know neighborhoods. 

@Kyle KnutsonArmstrong Plumbing and heating.

Surprises are not "hey we can't pay rent till the 10th this month", it's oh shoot the plumbing is bad in bathroom #2, that's a $4K fix. 

Looking to keep the first one too cosmetic, trying to stay away from major electrical, plumbing on the first one.