You don't need to take it down to bare plastic, you need all of the surface gloss off and just get into that other layer of paint well enough so it's got that soft fine velvet feel to it. Instead of sand paper try to find a cloth with grit in it, kind of like the cloth that you can use to polish aircraft windshields with. I think it's called micro mesh? It with fit the contours better and make a better product in the end. Start with a 600, then move to 1200, then do like the 3200 and that should be pretty good. It'll be a long task but the more time you take, the better it will come out.
You want him to go to 3200 before primer??
Sand to 600 then hit with primer. Get a flexible primer and try not to add too much paint or it will crack and craze. Two layers of primer should do it unless you're fixing damage.
Add paint, Two layers will be fine.
Fine sand, 1200 is enough.
Add clearcoat, Two layers again. Too much and it will not flex like it needs to on plastic.
Once it's had at least a couple warm days (lol) to dry, give it the finest possible sanding, use a rubbing compound for this rather than paper. Depending on how good you are at painting you may need to 1800 sand or higher if you can get it before polishing (rubbing). If you think this is going to be the case, add more clear to suit.
Painting isn't easy. You will **** it up several times.
Get some cheap broken panels for practise, it'll cost paint but save you so much time and frustration when you have to rub a panel back after a big run or a "dusty" coat.
Unless the panels are rough, they'll look much better with a few decals, the colour is too much on unbroken panels.