I wouldn't go that high, with loader $20k is about what it cost new... Any where between $17k and $15k, I'm sure the owner would be happy.
Now if you want to test to see if he knows what the tractor (actually all the markets) has done lately, you can low ball him from there.
I would check at least one thing, the L3700 was an in-between model (from
L3400 going to
L3800) I think Kubota was clearing out old parts before new parts were ready.
That's a total guess but I've seen where the
L3700SU did NOT have a suspension seat. Direct bolt on would jar your bones, just one thing to check to make sure it has one!!