The best way to blow a head gasket on an aluminum block is heat - likely from the water pump going out and leaking coolant into the oil. Make sure to watch your coolant level if you don't do it.
Mine has 113 and no signs of going out, but I'm going to have my brother do the pump (he's doing his at 100k as well) when it cools off some. I'd rather do the preventative maintenance and play it safe.
It was mentioned further up that you can't really tell when they're leaking and I found that to be true. I finally did mine at 245K and was very surprised how much pink crystalized antifreeze was in the back of the fan.
I ended up doing everything including the thermostat, radiator, idler pulleys, and belt. I had the tensioner but it was more PIA than I was prepared to tackle that day so it's on the shelf.
I'm a push it kind of guy but having a WP fail would not be fun on a road trip with family. I think 150K is probably a safe interval.
V6 215k miles original water pump, no leaks. did replace the radiator at around 150k.
Many, me included, just change the pulley if there's nothing wrong with the tensioner itself. You can either press new bearings into your current pulley or do it the "accidental" way I did - I bought a kit with the tensioner and the other pulleys then figured out, the same way you did, how much of a PITA it is to replace the whole part. Fortunately, the pulley and bolt off the new tensioner went on with no issue. I considered returning the kit and buying individual pulleys but no one sells one for the tensioner (that I could find at least) and the cost would have been about the same anyway. In fact, I'll send you my old tensioner pulley if you want to press new bearings into it. There are some threads where people talk about what bearings they used and so on. I didn't want to mess with it.
That said it appears the 4R water pump is really a non issue, so maybe at 200k+?