Category O is the very light one. I don't know the specs exactly to where it changes to a Cat 1, but it should include large garden tractors and such.
You can purchase it in the Market in the Vehicles category.
Your licence must include category K in order to drive a lawn tractor on the road. You do not need a licence to operate a lawn tractor on private land with no public access.
Category 1 refers the the size of the three point hitch. The pin size for your lift arms is 7/8". You will find Category 2 hitches on tractors above 50 hp.
A sub compact utility tractor is probably best for your small farm. You can find it here on the John Deere website: http://www.deere.com/en_US/ProductCatalog/FR/category/FR_TRACTORS.html
The three point hitch refers to the way ploughs are attached to an agricultural tractor. The dimensions of a category 1 hitch is 3/4 inch x 7/8 inch.
Generally in any part of the world where you need a license to drive a car on a public road you will need a license to drive a tractor on the road. It will vary between country to country, even state to state wether you need a special license class or wether the tractor falls under an existing (generally mass based) category.
John Deere provides a variety of farming equipment. They have their own website in the folling url: http://www.deere.com/en_US/ProductCatalog/FR/category/FR_TRACTORS.html.
You can always get a better price on a used John Deere tractor by selling it online. List it using this website, which will give you an idea of the prices. http://www.machinefinder.com/ww/en-US/category/used-compact-tractors
25,000 - 40,000 lbs. or more, depending on specifics. When you say "tractor-trailer", you're citing an all-inclusive category which would include everything from a single axle tractor pulling a 28' single axle trailer up to heavy haul units of over 20 axles... I've personally run combinations which have weighed in excess of 250,000 lbs. empty.
Thier cut off mark is 24 0
a John deer tractor
tractor