This is the worst way to lower your car. The spring is not the same all the way from one end to the other. If you cut it, you screw up the way it works and you will destroy the handling and safety of your car. And then what? You need to buy new springs anyway? You can't save money this way and all you get is a car that might look cool but drives like junk.

I know you might get somebody who's done it who will say that it's OK, but you'd probably want to go see their Crossfire and ride in it before you commit. You might find they either haven't done it, or find their car is a basket case and rides like a radio flyer wagon scraping a fart can all the way to Wal*Mart for duct tape.

It might be better to find a way to order the Eibachs (like on eBay) for $230 $US and have a friend in the USA send them to you. That's a lot cheaper than the $1000 AUD you're looking at. Or save your money until you have enough (For the springs, the shocks, the alignment, etc.) and bite the bullet.

Have you checked Old Man EMU (ARB) to see what they have? They probably don't do Mercedes SLK or Crossfire but it might be worth a check.

Don't forget when you change the spring size you have to deal with the shocks, sway bar connectors, front and rear alignment. This is not a lawn tractor! You need special tools and experience is a big plus.