When I was trying to buy a tractor, my experience was similar to shooterdon's. In my local area, the used equipment was either almost destroyed, or it was almost equal to the cost of brand new. I also looked for several months before I gave up and bought new.
A lot depends on your local area's economy, and how much equipment is available on the used market.
I would think a sub compact would do everything you need it to, but certain tasks like mowing the acreage, will take longer because the size of the implements you're using are smaller and can cover less ground in the same time. Same goes for tilling, or even moving dirt around. If your not in a hurry, it will work. Going up in size in your case mostly just gains you speed in getting tasks done via larger sized implements. Since you're not cutting and bailing hay, or moving large hay bales, everything on your list can be done with a sub compact.