Re: Preventative maint on Ford E350
Fender mounted starter relay. This goes out for me every 150,000 to the dot (well, within 1,000 miles of that). Good news it.... It won't leave you stranded. You can jumper the posts with a Leatherman.
150k is a good time to replace the coolant too.
Besides the starter relay, my only other failures have been the radiator at ~265,000, and recently my rear axle carrier. I still drove home 300 miles, with trailer, after the radiator burst. The Triton motors can actually drive without any coolant at all. It will shut off half the cylinders and pump air through for cooling.
I wouldn't pre-emptively replace anything other than the starter relay. The vans are generally pretty reliable. Much more so than the pickups it seems.
Carrying a large capacity jump-booster pack would my only recommendation. It'll save you from dead batteries, and if your alternator ever kicks it you can limp back on booster pack power. We use 1000-amp booster packs from car quest. We can get ~20 jumps-starts between charges, and have even used it on our Peterbuilt when the alternator on that went out at 11PM, about 40 miles from the shop.
2000 E450 dually V10 wagon