I just picked up a second 2000 Astro AWD back in July. It had just ticked over 100K on the odometer and runs well. After the requisite 100K maintenance items it is even better. The other 2000 Astro AWD (identical color, options, seating, etc.) has over 200K and is needing an intake gasket set. I couldn't pass up a pristine clean clone as a backup.
If you can find a very clean '04 or '05 it would be worth considering. All the weak points have been listed above, as well as the strengths.
Since you have so many miles on your '98 you already know all the ins and outs. Expect a transmission and engine overhaul within another 100K. If you are up for that, keep driving. A newer replacement will likely have issues surface as well. The AC failure may be the typical belly O-ring seal on the compressor - It's a common leakage point.