If the rents collected aren't at least 1% of the purchase price (i.e. 200k = $2,000 in rent) then not to bother you.

I know you may want to have all vacant units but why not collect some rent from these two units while rehabbing the other two?

If the contract is assignable and the seller would be comfortable with your assignment fee, you can collect a small deposit up front with the rest of the assignment due at closing.If the contract is not assignable or you think they would balk at your assignment fee, double close.

Maintenance, collections, and oversight will wear you down. 

These properties always seem like a great deal but usually if it's in this price point's typical environment, it could be a headache collecting rent or dealing with quality tenants.

A lot of collection agencies (at least around these parts) are actually law firms.

Since your seller is renting the home they will have the same issues, so you're compounding your potential of legal violations causing the entire arrangement to be tainted.I'd suggest too that you dig deeper in title and the name(s) of your seller as to doing the old finance with a small down, collect payments, foreclose rinse and repeat, illegal practice now, thank God!

that runs through a community bank's contract collection area.  

That way instead of making $0 they can potentially make up to 25% of whatever you collect from your tenant/buyer.

It's a collection of the best flipping info from the site.See you around the site!