The M6030 is a conventional gear-type tractor of very high design and build quality. On the 5030's the engine is a big 6-cylinder, and smooth as silk (not sure which engine is in the 6030) . Kubota had not penetrated the mid-size utility tractor market very much at that time so sales numbers were small - but they are premium-quality machines with no problems. Except for faded paint it might be as good as new with only 1600 hrs (comparable to an Acura with 70k miles) If it has had a normal life, it could run perfectly for another 20 years.
$14k - $18k might be an expected price range, depending on condition of tires, loader, clutch, body panels. Look for any oil leakage - there shouldn't be ANY on this model, unless it has been used hard. Any large leaks (droplets visible on the lower parts) would disqualify it in an auction situation - no telling how long it has been leaking and how well the levels were maintained. Also check the air cleaner and intake air tubes - if anything is missing or damaged, that's not good.
It's possible the bidding will be low - many people don't know what these tractors represent. Orchard tractors often look crappy (spray residue) but often have had a relatively easy life. You could get a great deal. A comparable new one would be near $30k.
The M6030 is big. The little L275 was also a nice unit, but way smaller - more of a subcompact.