Not hard at all. You will need the following tools :
17 mm socket
Torx male and female socket set

1. remove the fan ( 2 clips on top, connector on drivers side, wiggle thru the various hoses, none need to be disconnected )
2. use the 17 mm on the extruded boss of the tensioner to relieve tension and remove the belt.
3. the idler pulley has a plastic cover over the bolt, pop it off and remove/replace it.
4. the tensioner pulley assembly is attached by 2 Torx bolts, remove them and replace it.

put it all back together.

$154 for all three parts at Rockauto. Look under 2003 Mercedes SLK320 / Belt Drive / Belt Drive Component Kit .

Hope this helps !!!!