It's got a flat roof that I haven't seen replaced in the past decade. 

Depreciation is “baked into” home prices the same way appreciation is, and it tends to be fairly inelastic.

We used private money to purchase and rehab took 9 months, got it up and running in spring of 2017 and so far it's been good.It's a large 2 bedroom flat 2ndFL with two 1 bedrooms on the first floor.

If they are marketing and bringing you new properties, then a percentage or a flat finder fee for each property may be appropriate.If their role is purely administrative and they are simply managing the rentals once they are received, then I would only consider a periodic bonus based on the increased workload.

I go up slightly (my initial offer bakes this likelihood in) and if the seller counters again I explain why I can't go any higher (all good reasons - winter's coming and the gas bill will not be low, it'll take at least two months to rehab a unit/units and fill vacancies, etc.). 

Need quotes for a roof replacement on 2 flat roof duplexes.

Flat fee?

The entire 2 flat project with an addition at the top took me about a year. 

My math told me a quick sale at $400k would net me within $5-15k of what I'd net doing a master suite addition if values continued to trend flat/down. 

Flat, steep, treed, cleared, dry, major road, residential/ ommercial/agricultural, zoning, etc.