I wrote a buyers guide here which covers some stuff:
Mercedes W126 buyers guide. Covers the different models, series, what to look for, what to avoid, engines and most popular models to buy.
www.classicjalopy.com
What is your budget? That would have a big impact.
The 280 didn't fare well with the ADR regulations, so aussie ones don't like to rev. That is a problem as the M110 needs to rev, and so I would only consider a 280 if its a south african import. Apparently the 84 and 85 models are better, but I have no experience with those.
The 380 is a pretty good deal if you need a V8 and you don't want to pay 560 money. They are half the cost of a 420, and the 420 is not twice the car. Personally I would go with a 380SE if I could find one, the SWB versions are nicer to drive and the 380 came in both. Imports have more power if you can find one, but the aussie ones can be tuned up to have more power. Performance is pretty similar to the W116 450, although its less torque and more revs. Performance is not bad because of the 3.27 rear end. The v8's are pretty costly to run though, as most 380s have deferred maintenance as they are not worth a great deal and will need timing chain, guides, etc, and best case valve stem seals and worst case valve guides.
300 is probably the best value of the W126 range, feels much more sprightly than the 420 around town and the engine is the most modern. The M103 needs head gaskets, so its good to find one that is done. The early ones are more spartan, the later ones are quite optioned up. Very different character.
420 is good if you want a freeway cruiser. They are great for that with their 2.47 rear end. Personally I find them a bit sluggish around town, the 300 feels more 'alive'. The 420 is better fuel consumption than any of them, even the 300. Best year is 89, with the slightly improved interior. Avoid 90 and 91 models as the ASR is full of NLA parts. the 420SE would be the best, but they were only offered in 90 and 91, so you want an import if you desire a 420SE, otherwise get an SEL from a prior year. They were well optioned so not too much difference from a 560 other than power of course and the anti-squat, anti-dive. Same maintenance cost as the 380, but less deferred maintenance. If you want a pristine example 420 is the easiest model to find it in as many were sold privately to well to do buyers, vs the 560s that were often company vehicles.
