The PM presented us with another "bundle" of keys ... we then had 20 different keys for a 6-unit building.Needless to say, we now have eight (one for each unit, one for the mechanical room and a master) different keys.

We replaced all mechanicals, framing, drywall, etc!

Before you rent out the property be sure and replace anything that may go out in the near future...water heater, HVAC, garbage disposal, faucets, toilet mechanisms, supply lines to toilets and faucets, dishwasher, etc. 

The other replies hit on most of it: foundation, roof and mechanical/electrical issues cost the most. 

No matter which way you go , I cannot imagine that extensive or not full replacement of water lines, electrical and mechanicals such as a heating are not going to be needed or required by the city when a permit is pulled on a property that has been vacant for 10 years . 

At the end of the term of the lease, if there is more damages than the security deposit amount, you can still bill for the extra as long as it's reasonable and stemming from their use and not age of the property/condition of the mechanical systems. 

Which I let him have but knowing that in advance would have changed my approach to the project.If you are 50/50, you need to foresee these issues in the operating agreement and hopefully have a mechanism for deciding on them.

As you know, all you need to do is make sure the place is clean and mechanically sound.

Ask the age of all the Mechanical s - HVAC, HWH, Appliances.

So, what are the mechanics behind this?