There are other factors at play, but this was just the first thought.

All you'd need at that point would be RE attorneys for the contract and formalities.To find recommendations on seller financing deal structure, I'd start right here on BiggerPockets by using the SEARCH feature and playing around with different keywords i.e. owner financing, seller finance deal structure, seller finance terms, owner finance terms, etc.All the best!

That's probably not a pure parking play though.   

Would this be a slum lord play?

You may have a grasp on this aspect, but there are items to consider that can play a big factor when choosing your land such as topography, existing zoning & platted lot status, minimum/maximum requirements such as parking, detention, size of building, setbacks.  

If he's getting 10-12% return, why not borrow at 8% and play the spread?

Income over many years-weather plays a big part is the success of a wash, you want to see REAL numbers for st least 3, more like 5 or 7 years to get a better picture.Sellers mindset-is he really selling 2 of 3.

Landlords who pay attention, are ready to wait, play by the rules of the local laws, and have enough volume or cushion to ride out the outlier experiences, have seen, and I believe will continue to see, the benefits of lack of competition accrue to them to the tune of millions of dollars. 

Sounds like you did a lot of the work yourself which is always a bonus when you are trying to save money.

Is that the play?