A license will make it legal for you to buy/sell/lease property on behalf of other people for a commission.

That said, as someone who has done a few OF deals, the docs your attorney/title company will suggest you use depends on the type of owner financing: generic owner financing, land contract, lease purchase, etc. 

It is a tough call, we used to allow our owners to lease the properties when they were occupied.

As for the existing tenant, ask the seller to provide you with the lease agreement, so you know its terms. 

How would you structure lease so it won't effect value of building from less income.

Retail doesn’t want to wait for a lease to end and it may cost you 1-2k to buy out the lease for a sale... unless you write it into your lease.

When she questioned why the landlord feels that she needs to pay for the test, they sited the lease agreement where it stated that the landlord verified that there was no mold at the time she moved in and that it is the responsibility of  tenant to "maintain the premises in a manner that prevents the occurrence of an infestation of mold in the premises.

Only 3units have lease. 

The lease would have a provision that says the tenant is responsible for any repairs not caused by normal wear and tear.

Then you can establish new leases immediately upon taking possession.