Terry,
Bush hogging with 60" won't take any more than the 5103, in fact you could get by with as little as 30HP. On my farm, which has many set-out evergreens, I much prefer the 60" hog behind my
L4300 when mowing around the trees. The
L4300 (with loader off)/BB60 combination is very compact, maneuverable and pleasant to use; much more so than my larger Ford 4000 with heavy 72" hog. With a tree farm and a vineyard, I think you'd value maneuverability over size, muscle and weight.
I'm planning to hog the nasty stuff with a Woods BB720 (72") that is quite heavy. The JD dealer indicated that the 2WD 5103, properly ballasted should be fine, though I may still upgrade to a 5203.
FWD and a loader kind of go together, in my estimation. The loader needs the FWD for good traction (and braking) with a full bucket and the FWD needs the loader to fully justify the extra expense of FWD. Without the loader, a well ballasted 2WD should easily do 95% of what you're planning if your ground has good drainage. The remaining 5% is still doable with care.
But I'm not diminishing the value of FWD and a Loader by any means. If you can afford to go that way, definitely do it. Loaders are extremely versatile. Keep in mind, however, that you will likely want to be able to easily remove the loader for work around the trees and the grapes.
FWIW
Bob