I don't know if installing a new bag with an expended crash sensor will deploy the new bag or not.
I know that when I did this job I replaced the sensor before installing the bag. I followed that order as I didn't know what would happen and didn't want a bag going off in my face.
New, the sensor is somewhere in the range of $1200, which makes finding a salvaged one a good deal. A good sensor should not rattle.
As far as amount of work, it really isn't that bad. Strip out the stereo ACC and whatnot, then remove the center console and then it is just a matter of replacing the sensor.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.