I have a similiar size "lot" as you and I bought a 70 Hp cab tractor. Ideal for this size property. At the time all I could manage was a heavy duty 6' cutter. This is not what you want. It is very handy for heavy brush once, but if you want to cut the entire 100 acers you will need to start again about the time you are finsihed.

My recommendation would be at least 60 hp and at least 10'. Best bet would be a 75Hp cab tractor and a batwing.

Have you checked out the blue tractors? I tried out the M series and the TN series and in my opinion the TN is much superior.