I bought the cousin to yours (L3901) earlier this year. I have about 70HRS on it and so far so good. I've had two regens which were uneventful. Be very mindful of ballast when using the loader. These machines need every pound you can put on them in the rear. Congrats!
The dealer quoted $1700 for 3901 upgrade. Could not justify that. I think those extra ponies come from pump & ECU tweaks by Kubota. Besides my Iseki is 27hp, which was more than adequate for my needs.
Yes, filling tires & ballast box on the short list.