Re: Which engine for a beginner??
Considering you come from a heavy pedaling background, a 4-stroke clone would probably be just fine. The Huasheng (and Titan XC35 from Thatsdax) are based on an older Honda design, the GX31. They make great assist engines for people who aren't interested in speeds over 25MPH. Maintenance is easy to do on them and they are quite fuel efficient.
They'll do pretty well in taking the load off your knees. Expect an easy 20MPH cruise all day, all week.