JamesV6 - ensure you get a manual.
The price of diesel, an auto is too uneconomical IMHO. Even chipped (which done right, increases MPG (but breaks up MID MPG readings)), given the extra oil a tractor uses (6.5l) and 3k oil changes is the utter maximum, I reckon it (my auto tractor) wasn't far off the running cost of my MV6.
I loved my tractor though, esp chipped. Great fun. Remember, if you can't handle a big RWD car (no offence, but recall your laid back style), it has no TC, and (chipped) its ever so easy to powerslide it. Yes, you can powerslide a Omega Tractor Auto - I wouldn't want to see a post that it had caught you out, and no electronics to save you.
Obviously, unchipped, its about 100bhp down on what you have. Chipped, its about 70bhp down. Its an old style diesel, so a good wide range of revs with good torque, probably (chipped) about 1800rpm to 4500rpm. Oddly, for MPG, you may find a 50mph cruise is better by dropping a peg. I used to cruise my auto at 50mph in 3rd. Additionally, the MID MPG reading (inaccurate, but a guide (reads about 15-20% over when chipped)) was double at 80mph than it was at 40mph! (I think due to the excessive torque converter slip until you hit lockup at 50mph that plagues all TD autos).