Greg is right! Don't rush into an expensive repair. Haste got you into this mess.
Do a search to see if there is a good tech in your area. Check with the local MB club. Ask around. I'd start with a valve adjustment and while they're in there check the timing chain for sretch. But, take your time.
If it were me, I'd have my attorney write your "pals" at Loew's Shop a letter, as well as the BBB and call the Attorney General, they love to get involved with these kinds of things. Why should you be the only one losing sleep over this.
I'd park next door on a buisy Saturday with a sign in the window that tells everyone these guys ripped you off. (Check local laws first) I'd call them and say we can resolve this the easy way or the hard way.
Don't take this lying down!
91 300E 120K
90 300SE 275K (sold)
92 BMW 525iM 120K
90 BMW 525iA 175K
85 300D 175K (sold)
84 300SD 245K (sold)