I would say a Puch maxi with an E 50 motor on it is one of the best mopeds one can start with.... arguably the simplest motor to rebuild and maintain, and there are more parts available for it than any other moped, which I'm pretty sure also contributes to the fact that parts are very affordable as well.
It would have to be a really nice, really clean, low mileage and of course running Maxi to be worth $750 in my opinion. Post some pictures of your options!