Worst case scenario (if seals, compressor and hoses are fine) is that you need a new evaporator core. Cost is around $220 to $360 depending on where you purchase, and for what model. Only the compressor is more expensive...
The cost of the unit isn't so bad, but the labor is. Installation requires a complete removal of the dashboard and console, since it resides directly behind the front console panel. The evaporator's duty is to draw out hot air inside the passenger compartment, hence its location.
I had to have that replaced on my 300E a couple of years ago because mine had been also blowing hot air (less than a week after getting it recharged at the local Jiffy Lube). The final tab + labor came to $1800. Fortunately I had an aftermarket warranty that covered this, so I was only out $250.
Find a good air conditioning shop to troubleshoot the cause, but I would leave the installation (and subsequent dashboard plundering to the MB experts...Good luck!
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car