I've laid hardwood floors, tile (both on walls and floor), wired entire floors, plumbed entire flloors, framed entire floors, built a pole barn from the ground up, poured concrete, hung and finished sheetrock, etc, etc.  

Look it up, many investors have been able to implement this method for billing utilities to tenants and are happy with the results/savings/increased NOI etc. 

My favorite methods of finding owners who will sell that way, are by looking at the for rent ads and asking if they will sell; I also track expired listings through an agent friend and pitch them on a full price deal but on my terms.  

The space has central air and electric but the previous owner did not put in plumbing.

Replacing polybutelene plumbing is a hit you will need to take if you choose to continue renting.

I am looking to find new methods to help my company grow.  

I thought so too you could use the double close method to get around the whole "unlicensed" thing since you do actually own the house even if it is for a short period of time.

"turnkey" is supposed to be a method of buying a property in which all you need to do is "turn the key". 

I was given an estimate of repairs for $35 to $40k for total rehab. it needs roof & everything else like hvac, bring plumbing & electrical up to code, kitchen cabinets, appliances, floors, paint, etc.

I would second @Mindy Jensen's advice of starting with The Ultimate Beginner's Guide to Investing in Real Estate, it's a great place to discover the different methods of investing you might be interested in, then you can move on from there!