I know I will need new discs, pads and so on. My concerns are mostly as to if they will fit on the SLK caliper mounts and if the distance to the discs will be okey. I know some calipers was too far in to fit on the rotors when another member tried to do a brake upgrade.
I have looked at other big brake kits earlier, but just came across these nice looking AMG calipers and they were cheap as well.
Do not want to spend more than 3K for the fronts alone from brembo(overpriced)
So you want cheap brakes, then Brembo are definitely not for you. The SL ones you are looking at on ebay are Brembo, as are all SL models. Mercedes - and all other major marques - will only use Brembo.
You should be looking at the cheapo ones, eg:
Mercedes SLK R129 R170 R171 Front 356mm 6-Pot PB Brakes Big Brake Kit BBK | eBay
If you want to fit the ones from ebay, you will need a friendly MB dealer who is happy to give you parts on sale or return so that you can try various combinations until you find the right one.
The calipers you see are a Brembo standard that are used on MB, Jaguar, Porsche etc - they do not necessarily fit directly to the stub axle, but have a carrier bracket as an interface - my point No2.
Same with the discs, these are available in combinations of OD and offset. You need to find the right one.
Now, the ebay calipers say they are off an SL65. It could well be that the brackets and discs may well suit, but it will be trial an error - hence the need for your friendly dealer.
Happy days if you think you can get all the parts under £3K...!