Wow...you've got a lot of options!
First, you don't want to use a finishing mower over that 6 acres until you turned it into a lawn. If you do....you'll probably ruin a rather expensive implement.
You may read posts how some guy cuts brush with his mower, but it's either BS or he'll be looking for a replacement PDQ.
So, you'll want a rotary cutter to clean up initially. Go fairly cheap (King Kutter, Howse) on this as you'll probably only use it for a year or so.
As far as the 2 acres of finished lawn...I'd go for a pretty decent finishing mower (since you'll be using it for 8 acres in a year or so). With 52 HP (and I'd guess 45-48 PTO HP) you can easily go 84"-96". This is going to cost you 2200-3000 bucks depending on the manufacturer.
Once you've got that brush and such cleaned out, you may want to prep it for a lawn seeding. I would think renting a tiller and yard rake would be best for this. I'd rent, because you may not need those implements again.
Now you've got something to think about...I'm sure other TBNer's will be jumping in any second now...