Here's a drawing of an easy to build impulse turbine. The efficiency isn't great, but that's fine for now.
The alternator wants to spin at 3000-15000 RPM. Alternators are cheap. As long as it doesn't grenade every time you turn off the car, it should be fine without an overspeed brake. Getting it to spin fast enough under load might be an issue, though.
If you determine you need more heat exchanger performance, you many need to go for a bunch of copper tubes inside a 3" diameter exhaust pipe. Actually, I have a book with a chapter or two on analysis of heat exchanger performance, or you may be able to find a concentric tube-in-tube heat exchanger simulator online. But you'll need to know the inlet temperatures of the working fluids and the desired heat flux.