First, get the sucker clean. TSP, soap and water, etc. A good
detergent. Then sand lightly to "break" the current finish. Fill in any
dings, etc. Get the surface nice and smooth.
As to primers, you are likely going over an oil paint, so see what is
around for just that purpose, assuming you are going for a modern latex
paint. I've used outside semi-gloss paint as a primer, as it has lots
of pigment, and bonds well. A coat of that, and then a couple of coats
of your finish gloss, or semi-gloss enamel. You might be quite amazed
how much it is going to take to kill that purple paint! There is also
a wall primer for going over colored walls called KILZ, I believe, See
if they have a primer that would meet your needs.
Lowes or Home Depot are your best bests. I might even consider two
coats of primer, and some light sanding between coats to smooth things
up. Make sure things are DRY before you put on the next coat. At least
overnight, and longer if you are working in the garage where it is cold.