There are a few things to try/watch out for, depending on the materials involved.
I always dismantle the cars, partly to keep the windscreens separate and partly to get at the interior. If you're going with black windows this may not matter so much. It's also good fun, just remember to keep track of what goes where

With metal, it's usually enough to use matt spray varnish on the model. This tones down the "toy car" look and gives the weathering paints something to grip onto. I use really cheap craft paints for weathering, usually applied with a bit of sponge. If you do want to change the colour of the vehicle, normal model sprays (eg Humbrol) work. I have stripped some back to bare metal, but it wasn't necessary and the chemicals were very unpleasant.
With plastic, the big problem is the quality of the plastic. I have had primer bubble up off the surface and was able to scrape huge lumps off without applying much pressure at all. What I have found worked (recommended by someone on these forums) is the plastic primer used for patio furniture. It etches the surface and actually makes it feel slightly rough when dry. I use Valspar Plastic Primer. It's meant to be grey, but dries pretty clear, which means I don't need to repaint the model if I don't want to.
To stop wheels moving I've used both superglue and epoxy. Both work equally well.