but let me:
(1) Mileage - say that I wish you had given the mileage on this veteran car !?
(2) Valves - second let me concur that the valves adjustment is a good early bet for maintenance... especially since you do not know when they were last adjusted. In this regard, be certain you or whoever the mechanic is checks the timing marks to determine how much stretch there might be in the timing chain. Report this back on this THREAD.
(3) Tranny - As to the hard shifting, knowing the mileage is very important to know. Also please confirm whether this is a W123 or is it by chance a W126 chassis. I'm guessing W123, and if this is the case, you most likely have a tranny that might just benefit from an upgrade kit... but let's not get ahead of ourselves. It would be helpful if you could confirm precisely what 617 engine and what 722.??? tranny you have. In all likelihood your car uses what I would affectionately consider a god-awful vacuum control system, one that even many good and experienced MBZ mechanics fall short in understanding. Thus you need to let the people who read this FORUM know exactly what you have... engine and tranny #(s).

I might help some and I know there are others who know a great deal more than me for my experience thus far is limited to the 1980 300D I purchased for my Son.
Regards,
Sam