Hopefully this helps and also I highly suggest that you educate yourself more with MHP space as you may encounter few things that isn’t same as other multi family investment

The obstacle I'm encountering is that the [conventional] lenders I'm going through keep demanding 20% down (which I don't have---the money I have is being saved for use on rehab deals).

As a Broker the biggest problem we encounter is a license performing licensed activities outside of the brokerage.   

No, I’m not talking about walking down the runway in a ball gown…I’m talking about putting together a spreadsheet that allows you to enter a handful of variables like purchase price, renovation budget, closing costs, and other costs that we have historically encountered on our deals.

@Eduardo Beltran for what it's worth, I'm using PropStream and it allows you to set filters and you can see the loan info.I agree with @Don Konipol, the only sellers I've encountered who will entertain the idea of a sub-to are those seriously behind on their mortgage, or those who have no equity to pay a broker for a conventional sale. 

It has negative job growth over the past 12 months, and well as low median income, and high crime.http://www.city-data.com/city/Memphis-Tennessee.htmlhttps://www.deptofnumbers.com/employment/metros/Lastly, when building a portfolio of that many residential properties, probably the first hurdle you will encounter is lending.

Consider this, my SOP for on site staff who show properties for me when they encounter a brutal attitude reads: “give them an application say thanks for coming and tell them to put it in the mailbox when it’s filled out completely”... ie: push an app at them and get them out.

Have never  encountered this situation before.

I encountered one problem from the get go, I lived in California and needed cheaper properties to even consider investing. 

Also, for a fix and flip, who are some key players, you would want to have on your team (ex. roofer, painter ) I have tried meet-ups, but I haven't encountered many people who are willing to share their connections.