In the March 2008 issue of Bass and Walleye Boats they tested an Evenrude E-Tec 250 HO on a Ranger Z520 Comanche. The best mileage was at 3000 rpm. It was 4.2 mpg and 34.2 mph. Top speed was 76.2 at 5750 rpm.
As far as durability don't have the slightest idea. This should at least give your friend something to start with. Of course mpg and speed will vary based on the boat and a lot of other factors. The rigged weight of the Ranger was 2555 lbs.