A little more information than I have time to go over fully right now but in short,
The A155 EM front gear breakage, normal, common. Cam failure, normal, common. Discs lasting a long time, common. The A155 is a good valve for all of its flaws, it is one of the most durable valves on the market.
I wont even guess at a proper system replacement without you getting an updated water test for your well. This is important for many reasons.
900 vs 9100, no contest, 9100. The 9000 is the opiginal and has a brass body that has a tendancy to fail in the lower piston center section over time. The 9100 is a plastic body that we have yet to ever see a valve body failure on.
A-155 = noise. I especially like the loud slam when it goes between cycles. All systems will make some noise. The 9100SXT should be slightly quiter.
Salty water, the A-155 should not bypass salty water during regeneration. Something else is going on there. The whine may be due to a restricted drain line which can cause salty water issues.
What color is the valve body of your A-155? Yellow or tan? Can you post a picture of this classic system?
Once you show us an updated water test, we can easily recommend a proper system that will meet your needs.