You have to go by the PTO hp when your matching your tractor to a tiller. You have 16.2 pto HP. Thats going to limit the tillers you can choose from.
I like foward rotation over reverse. Takes less power and mixes in green manures better in the garden. Strongly reccomend standard foward rotation for your situation. (also they are usually cheaper)
Most of the smaller tillers have chain instead of gear. I've used both and other than needing to check the chain every year or so there isn't any difference that I can tell. Gear might be more quiet.
Don't worry about taking out both treads. Almost all the smaller tillers will offset to take out one track and thats all you need. I've had both kinds and currently have a Kuhn EL62 that takes out both my tracks, used to have a JD 660. Taking out both tracks is vastly overrated compared to having a narrower tiller that matches your tractors power to make tilling a pleasure.
Kubota no longer sells tillers since they have a marketing agreement with Landpride. Most all the tillers are made in Italy and then marketed by a brand. Land Prides are made in USA (more or less). Do a search in the Attachment forum for "tiller". Read all night.
With only 16 pto HP I would think that 50 inches would be almost too big. 42" with a little offset would be perfect. Brands I would buy are Kuhn, Landpride, Befco, JD,Sicma. Tomorrow call your local JD dealer and get a price on a 647. Don't worry about paying more for the green paint you'll get it back later when you sell it used or trade it in. Good luck and PM me any questions I might have missed.