If you're out of warranty, definately get an aftermarket intercooler. The stock intercooler is a Garett part made in Mexico with really horrible weld jobs and it'll break again. A month before my SLK32's lease was over, I was on my way to a second intercooler failure.
If you want to stick with the original part, you can order the core separately (look for the part number, it's probably covered up by the sludge mess). You can get that part for less than 1/2 the price elsewhere than at the dealership as a MB part #.
Also, with so much coolant in the intake system, you're going to get a lot of weird check engine lights. Mostly from wrecked catalytic converters. My entire exhaust emissions system was replaced a few months after the intercooler was replaced.
One final note, I had a huge list of problems on my car after the intercooler was replaced. Refuses to start, stalls randomly, revs randomly, accelerates randomly, random streams of white smoke out the exhaust, running on 3 cylinders, LOTS of weird stuff and ZERO check engine lights relating to the problem, ZERO error codes in the system. Only CEL was from cat converters, and when it ran on 3 cylinders it told us one bank stopped firing. The dealership had my car for 2 months while the service manager changed 1 part at a time from a new SLK32 on the lot and drove it home every single day. 2 months and 2000km later, it turns out there's a secondary MAP sensor near the throttle body, and it got dirty during reassembly giving incorrect readings and causing the car to go haywire.
