In all likelihood, your fuel distributor doesn't need changing. If it worked before the tuneup, it's still working. I'd first try to undo whatever this "mechanic" did. Look for loose or broken vacuum lines & fittings. A spray of carb cleaner will cause an increase in idle speed if it hits an open vacuum line.
After that, two things to check right away: duty cycle and coolant temperature sensor.
Check duty cycle when idling warm with a DMM with duty cycle function, $30 at Sears for a Chinese cheapie. It should flicker up & down if everything's ok. If it's steady, there are error codes built into the readings. Caution here: MB uses % off as their numbers, my meter (Sears) uses % on. So 30% on my meter is 70% in the manual. YMMV.
The sensor is the B11/12 sensor all the way at the back of the cylinder head on the left hand side (driver's seat view). Each of the terminals should have some resistance to ground. Both sides are identical, one goes to the fuel computer, one to the ignition. If one side is open, try switching leads, the ignition is less sensitive to temp. A new sensor is about $40. If the open lead was on the fuel side, you'll get an error code instead of a duty cycle reading.
I've got some links with more complete info at home, but you can search on "duty cycle" or "temperature sensor" and find recent threads with these links imbedded. Another on adjusting the EHF has these links.
Jetronic adjustment
General info, ya gotta dig to find.
Try finding what the mechanic did before spending more big bucks. It ran well with the old parts.
Editing to add: Check the resistance of the distributor rotor, it should be about 1kohms. There are rumours of some failing right out of the box.
Best of luck,