The flex discs are between the output flange of the tranny and the driveshaft and the driveshaft and the input flange of the differential. The two halves of the driveshaft are joined by a spline joint - no flex disc. If there's enough clearance, the driveshaft does not need to be removed.
The bearing and mount are relatively inexpensive parts - about the cost of the front flex disc. Labor is more, as the driveshaft must be removed from the car, the old bearing pulled off, and the next one installed. Maybe an hour labor.
There is no set mileage for that repair. The bearing can be a little loose and it still work fine and replacement may not solve any vibrations. As was the case on my '87 300D with 260Kmi (believed to be original part).
Its also possible that the centering sleeve (bushing) in the driveshaft (which centers it in the flange of the connecting device, in your case the tranny ouptut flange) lacks sufficient grease or needs to be replaced. I replaced the rear centering sleeve (bushing) on my car, but the front was still fine.