I would not consider myself an "expert", but I probably know just about as much of them as anyone else here.
They are pretty good mechanically. The white ones have poorer paint than most other colors, and since these rarely ever get washed or waxed, they will rust. Try and find one from the south. Mileage is a non-issue generally.
Biggest issue is "The Black Death", which is a situation where the injector seal leaks combustion, and over time a hard, tough, black crystal (almost like black plastic) forms up and over the injectors, down in the valley of the valve cover.
This can be a HUGE job to fix. There is luckily a special tool set you can get to remove the injectors with the engine still in the van (this ruins the injectors), but the further back from #1 you go, the more difficult clearance becomes.
Parking brake mechanisms get stuck (because morons do not use them). Related, the gear selector mechanisms get all loose and sloppy, and sometimes the little grommets pop out.
Fuse boxes go bad (bus bars inside get warped, poor contact). Easy to swap, under the steering column. Usually manifests as inoperative turn signals, but can cause other problems too.
Condenser fans once in a while, which if left not working will cause the A/C to cool poorly and eventually ruin the compressor.
Alternator clutch pulleys can seize.
Charge air tube splits, and the resonator tube on the turbo outlet can split. Aftermarket has a metal replacement tube for the resonator, so it will never crack again.
Glow plug control relay can stick slightly on, causing battery drains. Behind driver's headlight.
Rarely you see an external oil leak on the head gasket. Never gets awful, but is a huge job to replace.
One of the biggest problems with first gen Sprinters is they got treated like a POS Dodge, which they were not. So they got wrong oil, crappy filters, wrong coolant, etc. The ones that are well cared for hold up remarkably well.