You can do as you prefer, but if they are very limited I would leave them original. Just my opinion.

For most of my cars, I weather them or plan to weather them. I have put a lot of time into learning how to weather my models, houses, track, cars, locos. Some weathering can be done with a dusting that is mostly removable if need be, but it will never fully come off. They might not have the highest resale value, but I have seen amazingly well done weathered European train models go for more than the original price. Marklin resale market is big and there are lots of buyers, but you would have to wait longer to sell a specialized weathered car. I like to think of them as uniquely artistic items.

Its your stuff, if you want to do it, go for it. There are good books on weathering and lots of tutorials online. I only buy stuff that I want to use. There is only so much space in my city dwelling which I get accused of looking like a musty old museum anyway. Too many books and such as well!