An update for everyone. I decided to go the "low budget" route and opted to get a old (and I mean OLD) 6' Bush Hog I found for $200 (just needed a bit of welding).
I must say I'm impressed with what this thing can shred. I accidently found part of a small engine that was burried in the thick brush, sounded like **** but the thing was chopped into a pile of small parts before I could get the clutch in. The cutter was none the worse. I got about half the field done when the snow that was coming down started to stick. So the rest of it will need to wait until another warm day (probably spring). Once I get the weeds and brush under control, I'll need to plan an attack on the areas thick in brush roots. Thanks to everyone for your advice. It definately helped and I'll post updates to this thread as this progresses.