Are you sure the temp was not up when you drove it to the mechanics place? Look into that. And also from my experience, just because there is a coolant leak, there is not way the headgasket would blow. I remember I was working on a mitsu 4G63 (Evo Engine) and to check the compression dropped too much oil. And that blew the headgasket. So, from what I've read did you drop any coolant or a foreign object in the intake? Something might have caused too much pressure on the head. Replacing a headgasket is easy buddy, take the spark plugs off, and take the head off. Now, I've no idea about costs in SL but you might want to check if its really under compression. Start it up, and pull the oil dipstick out, if you see some smoke or fumes coming out of the dipstick then its definitely under compression. Another thing to do is put a drop. VERY IMPORTANT 1 drop of engine oil on the piston and see if it improves the idle.
http://www.aa1car.com/library/compression.htm
Let me know what happens.