I don't know about the rest of the country but our bookings are way up this year.

He would get 10% on his investment, then split the rest 50/50.

Rest assured, you are secure. 

You can rent out the rest of the apartments in the building ASAP. 

The bank would finance 75% of the purchase price, I would put $20k down, and the seller would finance the rest of the down payment for me.

Like @Linda Hastings said, many counties wait until "development begins" before an official street address (I.e., what many investors call a "911 address") is issued. 

Rest of post stands :)

Trying to purchase with big down payment and have owner carry rest of contract.

After posting a notice to vacate on the door, if the tenant agrees to leave, is there something I should have them sign to officially terminate the lease and legally take back the unit?

Evictions even when you can do them are not cheap; you might try working it out with the tenants first.There was a developer in our area who bought a 40 unit to renovate and reposition about 3 years ago; the tenants kicked up a fuss, politicians got involved and you can guess the rest.