At the minimum, I'd drain the oil/coolant/ATF and swap the filters for fresh ones - any good 15W-40, Zerex G-05/G-48(and it's Ford/Mopar/BMW/MB/Tesla repackages) and Dex-III fluid will work fine, stick to Mann/Mahle/Hengst for filters, purge the fuel system and see if the fuel in the tank has gone bad. Unlike gas, water does drop from diesel and it's a perfect environment for algae and other nasty things to grow. Drain a little fuel and see if phase separation has happened and if the fuel smells like it should. If not, drain the tank and treat it with a biocide. Running Lubro Moly's Diesel Purge to clean out the injection pump and injectors ain't a bad idea.
Check the vacuum system, Mercedes in the 1970s-1990s loved to run vacuum lines everywhere - the doors and trunk/hatch locked via vacuum and the transmission was also modulated via vacuum and a mechanical kickdown linkage. New battery is probably a good idea, unless it was put on a float charger to keep the SOC at 12.6-13V.
Once you get the engine running, see if the brakes work. At the minimum, flush the system out with fresh DOT4(Castrol works fine, ATE if you can find it). Might not hurt to pull the calipers and see if there's rust on the pistons and a wheel bearing repack isn't a bad idea.
Those old W123s might not be as sexy(i'm using the term lightly) as the W124 or W126 of that era, but they are legendary. They'll run up to 100% biodiesel or WVO in warmer climates with little fuss.