I would keep the W126, unless you have the money to fix electrical gremlins the W210 might have. There's a discussion going on in the diesel discussion forum about E300D's, which is the W210 chassis, and I think it had quite a few problems. The W126 is quite bulletproof. The M103 engine has a good reputation I think. But, I know more about Diesel engines than anything.
-Joe

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.