Have you checked out the Ody forum (odyclub.com)? There's a lot of great info there. It would probably be a good idea to at least check there for any past recalls so you can ask if they've been addressed. I know there were a couple for the 02, but I'm not sure about other models.
Do you have a trusted mechanic to look it over? Are they the original owners? Do they have service records? It's also good to know when the timing belt, brakes, and other regular maintenance was done so you'd be prepared for when those expenses will come up. And of course the obvious, has the tranny had issues/been replaced already? Depending on how honest they are they may or may not tell you if they have any concerns about it. By test-driving it though, you should be able to tell if it's slipping at all now.
From what I've read there's not a given mileage that tranny problems typically start at, it's pretty spread out. There are a couple things you can do to prolong the life of the tranny, there's a few good threads about that on Odyclub.
Our '00 is at 265,000 kms (164,600ish miles) and the previous owner said the tranny had just started slipping a couple months before we bought it. It slips and occasionally hard-shifts (clunks) between 1st and 2nd, but if we drive it gently it's not as bad. Apparently the tranny fluid will smell burned if it's getting really bad, so we're watching that.
There are some other potential issues that other 2nd gen Odys have had, we've seen some of them. Some of the dash backlights (odometer, clock, climate controls) tend to burn out, but it's an easy fix if you don't mind pulling the dash apart (really easy) yourself to replace the bulbs. There can be issues with the SRS warning light coming on (airbags), some people have been able to reset it or have found a loose wire, others have had to get the module replaced ($600ish). If it has power sliding doors, there's a potential for issues with those, but there's a lot of great info on Odyclub about DIY maint. and repairs if you're handy.
For us, the bottom line is that that any vehicle that age/mileage is going to need replairs eventually and we're prepared for that. We needed a van on a small budget and we'd much rather put money into maintining a Honda (we're not a fan of older Toyotas and Mazdas) than a domestic