Sandy soil is the easiest soil to till and puts the least amount of stress on a tiller. Chain driven tillers work just fine. The chains are enclosed in an oil or grease filled sealed case. The chain never contacts sand or anything else, for that matter.

No one can predict the life span of a tiller especially since you haven't posted a single detail that would indicate how much use it would receive annually. There are lots of excellent tillers out there made by Honda, Merry-Tiller, BCS and Ariens.... to name a few.

I have a T/B Super Bronco - 3rd season. It starts. It runs. It tills. It tills deep. Deep tilling in hard soil is tough on the operator - the machine wants to lunge forward - yes I have forward rotating tines, not counter rotating. Once an area has been deep tilled, subsequent tilling is an easy, one-handed, walk beside job. I have 3000+ sqft, of clay and shale. If I had sandy soil or a smaller garden, I might have opted for a smaller machine. My only complaint is the mounting of the air cleaner. This could have been made stronger. If you hit this hard, you could snap the mounting studs.
Hope this helps..
Jim