I agree that you should have this professionally done. Mustangs are routed for SS, and anything else will involve wood cutting.
I would recommend one of the singlecoil sized humbuckers for the bridge if you want to do it cheap without the risk of screwing something up. You could also try to buy two of those buckers, and maybe try to wire the neck pup so it'll act as two separate pups. Then you'll only have to cut enough wood for a 5 way.
Wait for someone else to add in on this before you try it. I've never done this before, but it's just an idea.