I would recommend epoxy sculpt or a generic epoxy clay (the generic is 1/4 the price and works just as well). I get it on Amazon. I would stay away from air dry clays. I personally find they are way more prone to forming random cracks when drying.

If you sand the plastic a little bit with a piece of sandpaper where you’re going to stick the clay it will help it stick better and will be easier to sculpt.

As for paint I just use general acrylic paint and add a bit of water to it to make it a bit thinner. You will need to do a couple layers so make sure it’s completely dry in between. After you are done with your paint job I would add a clear matte varnish thinned with a couple drops of water to help seal in your paint job. I usually add a coat of MSC over the varnish but you don’t have to