The seller just mentioned this. 

There's huge demand these days for desirable location.

Claiming health defects from mold exposure, along with a laundry list of demands.

It's off-market and I'm getting it for $85K, no interest, seller-financing. 

Seller finance could help you with this however I'm not versed on the lose of benefits/Medicaid's role.

Secondly, does anyone have a good metric for evaluating mid-term rental demand?

Which property has higher demand?  

Unfortunately, a lot of investors first starting out (myself included) tend to focus on finding motivated sellers in these types of communities and then try to talk to the park manager and/or owner to make sure they can do business there.

Unless the seller is dying to sell, or you can sense his/her willingness to depart from the property then I wouldn't say this is a go.

Patience, seller had contact issues and banker switched to two separate loans at the last moment.