Where to begin...
Prices for the coupes are all over the board and I'm familiar with your seller. About 7 years ago I found a one owner 1985 Lapis Blue/Palomino leather coupe with service records located in Orange County, CA with 90,XXX miles for $5,500. I missed the car by one hour. Your seller had it on Ebay within two months and IIRC it sold for $15,XXX. Here's one I recently missed by 24 hours: 300CD Archives | German Cars For Sale Blog
. I currently own two CDs a 1982 & 1985 but always looking for that 110% perfect one. High caliber original cars such as the OP seemingly purchased simply do not exist in our state or area for that matter.
So, for those of you in CA, FL and NY kudos to you for your ability to pick these babies up at Estate Sales for $3,000-$5,000. For the rest of us that have to buy from the flippers or higher priced advertised examples it's just the luck of the draw. Von and OldMako are spot on. There are so few really really good 123s running around out there in general these days. And sure you can find one for $5,000 but at the end of the day you will have $8,000-$10,000 in it. Been there done that!
One side of my family uses Birmingham Benz for service work. Great mechanic and knows these old Benzes. The other side of the family stayed with Crown for service after purchase. There are probably other good mechanics out there today as well. Are you planning to use a mechanic because of perceived lack of mechanical ability or lack of time?
Hopefully, you are too busy doing many other activites in your city/area: eating at one of Frank Stitt's many fine establishments, partaking at Dram's in their fine Bourbon selection, crab cakes at Fox Valley, jogging on the Mountain Brook/Lakeshore walking trail to take in all of the female eye candy home from UA, relaxing at your Lake Martin or Alys Beach home!
Make sure to call the mechanic back and offer to pay him to turn over the service records if they aren't with the car. Chances are he will send them at no charge. Been there done that as well...