No, it really isn't really hard at all to rebuild manual gearboxes. I have rebuilt two Triumph boxes, complete with Laycock overdrives. The issue really is parts pricing- parts for a Triumph box used to be fairly cheap ( whereas Benz bits don't seem to be)and it was impossible to find one in a boneyard that didn't need a rebuild. The benz boxes seems to be much sturdier and somewhat plentiful, if you are not in a big hurry and will accept an old iron case box.

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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)