If so, do you pay them a % of the rehab costs or a simple flat fee?  

Some basic numbers: you can enter the property market with flats in Czechia for as low as 15.000 EUR/USD in industrial cities but if you are interested in Prague you need to spend minumum 100.000 EUR/USD.

Flat roof repair.

Once you go over 4 typically you need a dumpster, also will need sprinklers and fire control panel that calls fire dept with any significant renovations.

Water/sewer are combined and are tough to get set up in a tenant's name, so they're usually just billed at a flat rate, or included in rent, but I have heard of companies making tenants get the accounts set up.

Your fee should be baked in before the 75-80%, but with the market trending flat to down, I know my self and other local investors are shying away from those numbers and looking at 70% again. 

I've also seen people fall flat on their face.

If you have an older house and need to add more Outlets/GFIs and a new receptacle for a vent hood or microwave, you will have to add breakers to your main panel.

Or you could also set the rate in flat dollars and have it only take dollar bills. 

As Theresa said, shingles are stiff in the cold, so may not heat up enough to expand out and lay flat or seal down.