I am attempting something very similar right now which is to swap the 3.2 in my 2004 Crossfire (Same as SLK320) with a 5.0 from a similar year CL500. I have managed to keep costs way down. I got a the engine from a rear totaled CL500 with less than 2500 miles on it for 1,600. which included complete engine with engne cover, wiring harness and Computer. the only thing missing was the water pump and accessories which can be swapped from the V6.

To put this engine in the Crossfire (R170 chassis and probaly the CLK) you would need CLK500, CLK55, or C43 exhaust manifolds. I am also getting rid of the stock collong fan and replacing it with dual 12 in fans.

the ECU is the biggest issue. I am having it Unlocked and then re-programed to the car with the star diagnostic system.