At 204K: mechanic replaced a motor mount and says the steering rack needs to be replaced.
At 175K: changed one fuel injector.
A few years ago I broke the ABS system while changing the front brake pads (I think I spread the caliper open (to make room to insert the new pads) too quickly, oops!) and haven't bothered to fix it (a pricey repair): the only way you'd know is the "ABS" dash light come on intermittently.
Driver's side passenger door latch interferes with gas cap door; this problem recurred annoyingly, but wasn't expensive to repair.
Otherwise, normal wear items only: timing belt changes, front brake pads and rotors, etc. No "sludge problems": I guess we got lucky.
We had this van almost since new (bought it from a family member who later bought a 2005 Sienna LE). It has provided low-expense transportation over its lifetime, has never left us stranded, and is generally a pleasure to drive.
I drive the 2005 fairly often and prefer driving the 1998 model, with 205K miles, over the 2005 with 40K miles: throttle- and transmission-response are noticeably better in the 98 and power is the same.
The *only* thing I like better in the 2005 is the fold-into-the-floor-rear-seats: it really gets to be painful on the back to remove the seats from the 98 to make room for large cargo.
The only persistent annoyance while driving is that the rear suspension remains inadequate; with two kids in back, it bottoms out on rough hits, and with 4-5 people in back, it's takes a lot less to bottom it out. I had "heavy duty" shocks installed a year ago and that has made no difference (so the problem seems to be caused by weak springs, not wimpy shocks).
I intend to keep this until its (first) major failure.
We'll be taking this vehicle on our upcoming vacations driving around the western USA.