Recently bought a 2001 XLS with 176,000 miles for $2,200. Fairly clean. Seems spunky.

It seems to have three issues I need to address.

1. I loosened the driver-side upper timing belt cover enough to look at the belt. It's severely cracked across the top smooth side of it. Possibly original.
I've looked at kits on eBay and Amazon that fall between $50 & $130. They have various amounts of parts. I'm sort of a "don't disturb it unless broken or can tell it needs replacing" kind of guy. What gaskets, seals, and parts should I certainly replace on this Montero? Do I need any seals or gaskets that won't be found in any kits? Do I need any intake manifold gaskets? Like should I replace the tensioner wheels, hydraulic tensioner, etc? Or just change the water pump (its gasket) and belt?

2. Dash temperature gauge on this Montero fluctuates from stone cold to 1/3 upwards, almost halfway, and back to cold as I drive it. It initially reads cold like it should but then after several minutes of driving it moves up and down. The AC cold works fine. The Heat settings blow lukewarm but never gets hot. I've never had a thermostat cause that gauge to move around like this, have you? If coolant sensor was bad it wouldn't keep the cabin air from blowing hot when demanded, would it? I want to replace correct part.

3. The SRS airbag light is on. I don't see any tampering evidence. What does this typically mean?

Thanks!

I would either get a timing belt kit from Rockauto, this should include the water pump and if it has one a belt tensioner, that or one from the dealer... don't really have an opinion on quality, but timing belt letting go on these interference engine's will kill it. wouldnt really go for one from ebay, but that's just me.
Temp gauge, my first guess would be that it doesn't have a thermostat, or the T-stat is bad... most likely, gauge, should be easy to check, but im not familiar with that model.
Srs light, on the 01 you should be able to use a scan tool and pull the codes, then go from there.
Hope that helps a bit.