painting shells hmm...
It depends on a few things are you good with spray paint? you will need to make sure any paint you get is for Lexan, and other polycarbonates. All too many times I see someone buy the wrong paint then start wondering why it started falling off, and looks like complete crap.
Now if you are not good with a rattlecan you can order Parma Faskolor (not sure if its available in the UK, but there probably is a similar paint) that comes in 2oz bottles. You can either use it with a spray gun, or use brushes with it.
My 1st few RC's I used it with a brush(the picture below is of my 2nd paint job with a brush), and honestly they came out great IMO, it just took a long time cause I put on like 6-7 coats to make sure the colors took with like 20-30 mins between coats(I also did tri color). You just have to make sure you mask off what you don't want painted with masking tape and take your time.
As for light decals yes they come with them. Also the unpainted bodies might come with light buckets you can use instead. I know my HPI Nissan 350Z body did I painted them with Faschrome (what Parma called the color), and used shoe goo to glue them into the shell after I was done painting it and let the paint dry for 2 days. Then put LED's in them so they light up
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As for the arm personally I'd replace the arm way easier to sell something that needs 0 work then something that has a broken part. Most times when people put something is broke on an item for sale(specially a part easily changed, and is cheap) I usually think if they were too lazy to fix that what else could be wrong.