I agree with the others and get a lawyer to write it up.

The first step for me would be to introduce myself to the tenants - in writing - with instructions for payment of rent, reporting maintenance, and a request to schedule an in-person inspection of their unit within one week.

If they are VERY professional, they will have their processes in writing as verification that it is enforced equally and fairly by their entire staff.6.

I would like to write a contract to purchase the property contingent on approval to change the property's zoning (has to go to Board of Commissioners, takes about 60 days). 

If the old tenant is still getting mail at the old address, tell the tenants to write "return to sender" on the envelope and put it back in the mailbox.4.

Purchase price: $199,700 Cash invested: $39,700 This is a colonial style building that was originally a single family home converted decades ago to a duplex.

So you would not have to write it into the purchase and sales contract, you could create a separate agreement that descibes the two week provision for 5 years for the seller and include it when you present the purchase offer.    

I've always wanted the freedom to travel the world spontaneously, and I think taking some business trips to my STR's all over the world would make for a great tax write off. 

The guy still needs a place to live so I'm thinking about writing a 3-5 year lease into the contract.

DIY you can write out all the terms and conditions on a yellow pad and both sign it.