Bill says:

First clean them up with soapy warm water and rinse all soap off. If you want to repaint them you will have to bring them back to the basic plastic or remove at a minimum the reporting marks. That can be difficult. I have had success with rubbing alcohol or sometimes paint brush cleaner. go slowly and try to remove only the marks and not the adjacent paint. Once clear of all marks prime the car body. You now have a blank slate. Paint the colour you desire then apply a high gloss clear coat. Then you are ready to apply new decals and follow up with a dull coat to seal the decals and hide minor blemishes.
If there is still a noticeable elevation difference between the car body and where you removed the reporting marks you can smooth out the area with a very fine sand paper (600 grit or higher). Do this prior to applying the primer.
As for weathering that is an art and I advise you attempt your approach on one of your original cars. There are numerous ways of doing this from powders and pan pastels or air brushing or oil based paints sparingly applied in small spots along the top edge of the car then drawn straight down with a flat bush saturated with solvent. I am sure there are other ways but those are the ones I use. Go online and see more precise directions on how to do it.
Good luck.