The diff plug is 14MM. On the diff you can get to both of them pretty easy, the one on the transmission(I'm assuming it's a manual, an automatic 240D is not a good combinations) is difficult to get at.

The shut off problem is undoubtedly vacuum related, the vac problems are very easy to troubleshoot and cheap to fix, it will usually be a soft line rotted or a door lock diaphragm leaking. The door lock diaphragms are about $10.

On the 123, the Haynes manual is the best information for the money, it is a very good manual on these cars.

Good luck,