You can get a rehab only loan from a number of hard money lenders depending on the loan size or refi with one

Depending on your approach and the unit size you can be anywhere between $2,000 and $8,000.

So you need to decide a size range, and level of finish out you want. 

With the appreciation, tax benefits, offset mortgage, etc. the upfront costs in undertaking a good sized project are minimal compared to what is possible 5,10,15 years down the line...Just something to keep in mind

The average rental for single family home my size and area is $1350.00. 

He says a roof that size would be estimated at 20k.

It can run in the $2500 to $10K depending of the repair, unit, size, etc. etc.  

There's no one size fits all, but it's best to consider several options!   

What's the size of the property?

Of course that depends on insurance, deferred maintenance, size of park, what you work out with the onsite manager, taxes, etc...