Get both, a R107 for the nice days, and a 123 diesel for dayly driving
I have both and, and after a little maintenance both have been highly reliable. My R107 is a 84 280SL with a 5 speed manual. It is a blast to drive on twisty mountain roads, with its high revving M110. The main maintenance so far other than appearance mods, was to replace valve stem seals, and adj valves.
The 78 300CD has 250K on it. 10K put on by me in the last 10 months. It has taken alot more maintenance, water pump, new starter, up-grade battery cables, adj valves, and replace most of the rubber mounts. Car has never left me stranded, and if you do your own work these repairs are really not more expensive than on any domestic car. I like the simplicity of the OM617 engine, the sound of a diesel on start up, and being unique. The non-turbo is geared perfect for our gravel/secondary roads here 60/65MPH, but is to low geared for interstate speeds 75+. Looking for a second W123, a turbo 300CD for interstate cruising, with its higher gear ratio's right now.
Both the R107, and W123 are classy, raw, and just fun to drive. Plus easy to work on without all the modern electrical BS. Do not think you could go wrong with either.
Right now I am restoring a 72 Chevy K10 4x4 Good Luck in your search