yes the acumulator is suppose to be replaced when you open the system up. now if it is for a short time then it should be all right.
But the accumulators often rust through so if you are spending the money on the A/C system you might just want to replace it anyway to save money down the road if it where to rust through.
as for the system, it should be evacuated and the R-134A captured as it isn't legal to just vent it to the atmosphere.
fyi 850 doesn't sounds that bad for a shop to do the work. It cost me about $600 a few years ago to have my a/c system checked and the accumulator replaced as it rusted out, recharged, etc at a Ford dealer.