No need to take it off...as long as the compressor is just coasting, it uses almost no power to turn it. You'd never notice the difference, and it would screw up the tension on your belt, you'd have to go on a quest for a shorter belt size to fit, etc., etc...more headaches than it's worth.
That is a lot of $$ for air conditioning, but MB is notorious for unreliable climate controls and expensive repairs on them. I wish they built the climate controls to be as bullet-proof as thier diesel engines are! How good are you at Do-it-yourself? Does that $2,500 include labor? You can probably do it for MUCH less yourself, probably half that amount, if not less. You can also really cut corners with rebuilt/used parts, but obviously you will sacrifice some reliability, and any warranty that you would get on new parts. If you have leaks in the AC hoses, you should seek out a shop that rebuids them...it usually costs a FRACTION of what a new MB hose assembly costs.
Hope that helps,