I've been reading great things about the express model, but I haven't found a solid estimate of when to expect standard wear & tear to claim the transmission or the drivetrain.

There's a craigslist ad for a 2001 Chevy Express 3500 @ 175k mi, so I'm just wondering how far down the line I can expect to need to have those parts worked on.

 

That all depends on how it was used and maintained. Take it for a test drive and see how it shifts. Pull some fluid out on the dipstick, smear it on some white paper, and see what it looks like.

 

A 4L80E is what I believe is in them. They're a tough unit. I drove one 100 miles blowing fluid out a bad leak, stopped every 30 miles or so and put a quart in, and never seemed to bother it. If they get low they'll hesitate the 1-2 shift or slip a little. I would expect 300,000 miles out of one without any serious issues. As long as the fluid is kept at the right level. I've seen running and driving examples with much greater stated miles but no way to determine if they have the original transmission.

Scotty Kilmer says do not flush them when they get old. The fluid carries some particles in it that the transmission needs to work.