Here's my 2 cents:
I had same decision about a year ago but with only 4 acres. Ended up buying a new Kubota
B2910 HST with FEL and backhoe. (Around here the only used tractors that aren't older than dirt, if you can even find them, have either been beat to death or are almost the same price as new.
What really convinced me to get a new one vs. an OLD one was one horrifying night sitting down and reading many case histories of tractor accidents on the web. After that it scared the hell out of me to think about using a tractor without a ROPS, belt, PTO guard, safety interlocks, etc, etc. Even with these features things can get out of control real fast and you or someone else is maimed or dead.
If you can at all swing it financially, don't fool around. Get a new (or used late model) name brand tractor from a reputable dealer. Make sure both of you are comfortable with the controls and getting on and off the machine. Give them a work out with typical tasks to see how you like them, for example demo the FEL/backhoe on a pile of dirt (a lot of dealers have one for this purpose).
Bigger is often better, but not always. For us, the
B2910 is about the perfect size. Any bigger would be tough to get in the round pen and around some of the stable areas. Also, it was easier for my wife to get on and off than the next larger frame model up. Yet it has more than enough power for digging holes for planting big trees, trenching, moving hay, rocks, etc, at least for us.
Don't forget to consider the attachments you will need for your situation, often they dictate the tractor needed. The dealer probably has a lot of experience fitting the tractor and attachments to the job, so consider his/her advice (ours did).