I have an ATV and a garden tractor that I use for some similar tasks. The Garden tractor is much better at most of them. It turns sharper, can pull slower, easier to get on and off of, better parking brake, etc. The 4wd ATV will pull a little more, but isn't great for that type of work. I also have a B-series Kubota tractor which is the better than both for garden related tasks.
Depending on your budget I'd go one of two ways:
1. Small compact tractor. You should look for either a B-series Kubota, smaller compact Deere's (650/750, 655/755, 670/770, 4100, 4110, etc), or some of the smaller old Ford compacts 1x10 or 1x20 series. You only need one with 2wd for that type of work, but 4wd is a bonus. They can use a huge variety of gardening implements, can cultivate decent sized crops pull a good sized trailer, etc. You could find a nice used one for $3000-8000 depending on the year and model. It will use widely availble cat. I three point hitch implements.
2. Large garden tractor. This can cover a wide variety of models and years. You want something that can power a tiller (if you get an older model you better get the tiller with it), pull a cart, maybe some other light ground engaging tasks. I have a 265 Deere which is about as small as would be useful in a garden. It can run a rear mounted tiller, although it isn't great at that. Something bigger with a small 3pt hitch or similar setup would be better. For a used garden tractor with a tiller you are probably in the $2000-5000 range.
