I plan on walking the property with a contractor before placing an offer with the intent of scoping out the needed work on electrical, plumbing, roofing, and the foundation, which would be the big ticket items, along with all other work.

They may have to move some plumbing connections around but it shouldn't be much more than a regular water tank.I understand that some systems have a water heating system separate from the hot water. 

Few weeks back I asked them if they had any issues with their property including plumbing and we found a leak under the kitchen sink that apparently was there for a while (tenant never reported it to me) and now I am arranging services to make the plumbing repair. 

Moisture / water / mold, knob & tube wiring, foundation issues, ancient plumbing, rust on the breaker panels, termite tunnels, powder post beetle damage, cedar post supports, improper footings, temporary lally columns, etc.As to writing an offer, that's a sensible and common approach. 

All in with raising the floor for plumbing and then adding a small bed loft above it I paid 22k and that was with furniture and appliances.

My understanding is the bathroom is like for like assuming you are not moving plumbing.  

Roof, flooring, walls, plumbing, windows, etc.

I would suspect the buyer has a background in plumbing so they are sensitive to this, or they have had a property with this issue in the past, or they are simply making noise with the hopes that they can get a price reduction. 

First see if it's possible to separate the plumbing for each unit so each unit can have their own meter and account.

I can use them for plumbing electrical work sheet metal work or just holding something that my hands aren't strong enough to hold. channel locks are a must-have. if you can't think of a use for them just have them and you'll find one.