[sheepdog 44]

Some generalizations, (because i've also asked this question before). You want an electric motor sized for your car. The 25 hp motor will be able to give the constant hp needed to cruise. It'll be rated 25hp max, and such and such for constant hp. Where the 15hp motor may have heat issues etc and be inefficient driving through the whole range of power needed for hills etc. The motor with the constant hp rating needed to cruise at 65mph is the best choice. Bigger or smaller hp motors are not more efficient than the other like an ICE.

Are you planning on making it an EV? Depending on how efficient the platform is maybe a small motorcycle engine will do. You could make it modular to drop in or take out of the car depending on the expected range you'll travel. But the total weight and space of the engine may not be that big (Unless you still want to abide by hypermiling principles) to warrant going through the trouble of making it modular.

You will not be getting 150-200mpg on gas in a serial hybrid. With all the losses of gas engine, generator, storage, motor. I don't know the scale of your car, but if it could do 100mpg on gas like the Edison2, it'll only do 44mpg running on gas in a serial hybrid form. Which is why a plug in Prius gets 50mpg on gas, and the volt does 35mpg or so on gas.

It would be better efficient wise to have the engine directly driving the car. And manually stop EV mode when you have 10miles left. Just use the remaining battery power to accelerate with (or without) the gas engine. Only assist the gas engine when accelerating, and you can get away with a more efficient small displacement gas engine.