You should be able to verify the bad ring gear yourself by inspection underneath before starting the job. Yes removal of the transmission or unbolting and sliding it back is required to replace the part. Rather than replacing just the ring gear change the flexplate and gear as one.
A used part should do fine and with a little luck and no experience you could do the job on a saturday for example. This is not rocket science with the guidance available on site.
If the part used cost say 25.00 will you work a day for 725.00? Under a pit or hoist even the first time could go faster. Use new bolts on the flexplate to crankshaft though. You will need to buy, rent or borrow a torque wrench to do it properly. Getting a friend to help never hurts.

I have a couple of automatic flywheels available if anybody decides they need one.

Tom W

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Before you spend the money you might want to look at the whole ring gear and see if hit has burrs on the teeth.
I have had at least 3 situations where the ring gear teeth had the metal burred and the burrs were preventing the starter gear from engageing. (Done with the eingin in place and car/PU truck on jack stands.) I used a file and file and went around the whole ring gear and restored the teeth to as close as possible to normal and this fixed it. There may not be as much room on a Mercedes to do this with a file but maybe a Dremel tool of die grinder might work.

__________________
84 300D, 82 Volvo 244Gl Diesel

Another reason that a starter solenoid might not work correctly is if the wire going to the solenoid has a lot of resistance in it due to corrosion, part of the wire broken or burnned, or corroded contacts. Of course band ground to the engine. Try using a remote starter button connected directly to the solenoid wire and see if the solenoid ingages properly.

__________________
84 300D, 82 Volvo 244Gl Diesel

My starter on the 84-300D makes a high-pitched whinning noise during starting, but it does engage. I just put in all new glowplugs so it starts fairly easily. Does that like the solenoid going bad?

Don

__________________
84-300D 256K
81-300SD 217K (Wife's)
79-300CD 212K - Project
89-Volvo 245 305K

Thanks to barry123400 and Diesel911 for your replies. My much more car savvy friend and I will take a look at it this weekend. Much appreciated.