I have seen some very nice weathered models and have considered doing
this myself but it would be a lot of work and could easily go wrong
and ruin your model. I have started weathering my wheels to good
effect as bus wheels are never that clean. As for buying such models
off the shelf, again it would depend if they got it right. Corgi have
a lovely series called Hidden Treasures - ie old cars and vans with
lots of realistic weathering, rust and repairs. In short, yes it
would be great and would add lots of realism to a diorama, though
perhaps not to my display cabinet!

Mark.