Re: Soon to be FL Rider! Looking for some advice.
Sounds like a stock bike. Disregard the HP rating entirely. It'll probably go between 25 and 30.
MPG will realistically be between 60 and 75 MPG, but that's for premixed two stroke fuel, so your cost per gallon is pump gasoline plus oil. Works out to roughly twice as much as just pump gas. So, since your fuel cost is higher, the cost per mile (in fuel only) ends up being about the same as a subcompact car that gets very good gas mileage. Of course, you don't (usually) have the added expense of insurance and registration, so that helps.
Before you buy a machine that someone else has built, look it over carefully. Make sure the rear sprocket is straight, and in line with the engine sprocket. Make sure the chain isn't slapping all over the place, and that the tensioner (if there is one) doesn't want to fall off. I would want to ride the bike up to operating temp (which could be five or ten minutes), and then check the spark plug. Check that the engine mounts are flush and solid against the frame. Ask if anything has been upgraded - spark plug, coil wire, fender stays, hardware and fasteners. Ask if the head nuts have been torqued, and if so, to what tension?
$400 for a stock bike build like that sounds reasonable, depending on what frame it's on.