Hi,
Some good info here:
For light cutting anyhting from a basic 500W one will do, keep an eye on the sheds/Screwfix/Argos etc for special offers.
Slitting discs are very handy but easily damaged, so get a few of those. Once worn to a smaller size they're handy for getting in tight spaces.
Best to practice on some scrap metal before doing anything on the car!
cheers, Pete.