I have a short list of Kubotas I'd Like to Own (KILOs?).
That list is mostly populated by the largest 25-35 year-old L-series tractors.
I developed this list by researching the specs, dimensions, and engines at tractordata.com.
They have a 'full' listing of Kubota models, here:
TractorData.com - Kubota farm tractors sorted by model
At that site, you can also learn how models transitioned over time.
The data is not 100% accurate and often incomplete, but it's a good starting place.
Once I had my short list, the next thing is to see what you can learn about the marketplace.
A great place to search previous and upcoming sales is tractorhouse.
Here's one of my typical searches:
KUBOTA Tractors Auction Results | TractorHouse.com
You can play around with the selection buttons on the left to search for what you're interested in.
Under the "Listing Type" button, I'll look at Auction Results to see historical data, and then Upcoming Auctions and Sales to see what's up for sale (and where).
What I have noticed over time is that the sales prices of tractors sold at auction are highly variable, and not always driven by condition. The market can be more or less active depending on region. Attachments and features (Dual Traction) often increase the apparent demand.
I generally like to 'kick the tires' before I make an offer or a bid, but I bought my G1800S sight-unseen, two states away on an online auction based on its low hours (<1200), and its selling price (about half what I was willing to pay). Bidding remotely (online) is a bit unnerving, but not so bad if there's not a lot of bidding activity.
The two tractors I have bought at auction were previously owned by people who tended to neglect regular maintenance or minor repairs, and I filled a kind of 'rescue' role in buying them, 'just in time'.
Good Luck!
-Paul