You are getting reasonable quotes for rebuild/repair. Even if you find the adsit rebuild, you are still on the hook for a number of hours shop time. Comes to $2500-3k.
One very good option is to tear it down yourself and do the part replacement on your own bench. Should run $300 for parts. IF you have access to junkyard cars, you can source the transmission from a R107 (73-75) or the w114/115 after 8/73. W116 will have the same transmission. 3 speed with torque converter - W3A 040
One very good option is to tear it down yourself and do the part replacement on your own bench. Should run $300 for parts. IF you have access to junkyard cars, you can source the transmission from a R107 (73-75) or the w114/115 after 8/73. W116 will have the same transmission. 3 speed with torque converter - W3A 040