Those GM truck LS engines and the related parts seem to do quite well with high miles, IMO... as long as they've been somewhat maintained. I sold my last truck with 300k+ on it and it was perfectly reliable. I've driven work trucks that had 450k+ miles on the original engine & transmission!
A water pump will usually start seeping before it goes all the way out, so you have some warning before it totally dies. I wouldn't be too worried about the fan clutch or the alternator. You could buy a new alternator and keep in the van as a backup, just in case the current one dies somewhere remote.
Plugs and wires, yes.... coils no. I would wait on a coil failure before replacing them. They usually last a long time and if one dies, it won't leave you stranded. That and replacing them isn't cheap at all, so I wouldn't worry about coils.
More common internal failures on GM LS engines that I'd think about would be replacing the oil pump, timing chain & sprocket set and the roller bearings in the rockers (swap in bushings). But that's a pretty major internal work.
I'd do the easy stuff mentioned and don't worry about the bigger stuff unless there's a problem.
