31 years living in a maritime climate; as chipchop says rust is the enemy so check everything. 124s at this age will be hiding it potentially even under the underseal. Jacking points, sills, suspension mounts WILL all be suspect if not addressed already so look carefully. Can you get the car on a ramp to examine properly?
Suspension rubber will probably be perishing due to age; the plastic joints in the vacuum system will also be hardening and cracking and the tubing will be going the same way. That could be affecting the gearbox changes.
The S124 OM606 24v I had was slow to go but alright once wound up. I seemed to average 34mpg everywhere. I've no idea how much slower the earlier 603 is and if it's any more economical.
After only 90k the interior should still be in good condition unless it's been left in the sun all day, every day but at that mileage it seems unlikely. Despite it being of a build quality you just don't get these days things may still need replacing so just buy replacement bits off the internet.
Sedate, wafty driving is the order of the day. The rubber for 15" tyres is cheap and will absorb some of the bumps of modern roads better than the skinny tyres of today.
At this age you have to be prepared to spend money on the vehicle unless you are handy with the spanners and have the time/inclination. That said a well maintained 124 is a thing of beauty so good luck!