I don't do Ac work or rewiring of electrical. 

I think a company that does HVAC could give you details on correcting the CO issue and a rough idea of the electrical work required. 

He is used to doing everything (plumbing, electrical, countertops, cabinets, etc) himself, but smart enough to ask for help when necessary. 

Plumbing, electrical, and updates to the kitchen and all the baths?

Take the rents and minus the required expenses (trash, water, sewer, community electric, taxes, insurance… (Don’t include your mortgage, if you will have one).8.

All of the utilities are being paid in full at all properties with the exception of 3 units where the owner is paying water/trash/sewer and 1 owners meter electric bill.  

This will keep your equipment in good shape, make it last longer, will uncover potential problems, and may potentially keep you from a costly night/week-end call-out.At minimum, this check should include about 20 or more items, checking the capacitor, amp draws of your fan, compressor, pressure check, air flow check, inspection of electrical components for corrosion/pitting, and many other things. 

But if tenants are paying heat, then you will just have water and sewer plus any common area electric, and those should be much less than $150 per unit.

Common electric?

Has all the plumbing and electrical been updated?