The question is....... do you actually NEED a garden tractor?
There are plenty of LAWN TRACTORS out there that will cut grass, blow snow, plow snow, pull a lawn roll or garden cart but they are not suitable for ground engaging work in the veggie garden. So, the first thing we need to know is whether your veggie garden is large enough and situated ideally to make use of a garden tractor.
You may be much better off to buy a walk-behind rotary tiller instead. A garden tractor is a large machine and it takes quite a bit of space on all four sides of the garden patch to turn the tractor around so you can make the next pass with the tiller or the harrows or the turning plough.
We also need to know what your budget is because there is a huge difference in price between the $2000.00 MSRP for a John Deere L Series lawn tractor and an X700 Deere garden tractor that costs 4 to 5 X that figure. Yes... I know you want to buy used but a used Cadillac costs more than a used Chevrolet.
Aside from the garden aspect, how large of a property do you own? How much grass do you need to mow? How much driveway is involved if you are in the snow belt area? What other tasks do you need this machine for?
The more information you give us, the better we can advise you when it comes to brands, models and attachments.