First of all, there is a major oil change at 400 hours, can you talk to the previous owner to see if that was done? You are planning to get the manuals? They do a good job explaining how to do the maintenance. Only thing I don't like in the maintenance schedule, kubota doesn't list hours to replace thing... on the far right side they say replace yearly or something. Find out what the previous own has changed.
The return hose from '04 to early '06 didn't last long, they would crack and allow air to be sucked in. If the power steering or loader acts funny or slows down... that's usually the problem. My guess is that has been taken care of. That's the curved 1.5" (?) hoses by the brake pedals.
The sight glass is "weak" or some were over torqued at the factory. I've never had problems with mine.
One last thing that everyone should check is the oil in the front axle. Mine was down 2 quarts!!!
http://www.tractorbynet.com/forums/kubota-owning-operating/201623-l-series-front-axle-service.html
Also, Almost forgot, the suspension seat tends to stick in the down position...
http://www.tractorbynet.com/forums/kubota-owning-operating/129978-l3400-2800-suspension-seat.html