No reason a used gearbox should be any less reliable than one in any of the other thousands of C240/E240s out there.
I reckon if you hunt around, you should be able to pick up a good used box for £300/400ish, especially as W202s are worth so little these days.
Find a competent garage to swap it for you, shouldn't be more than a few hours labour.
Maybe around £750 in total would be a realistic repair cost - half the price of a reconditioned box.
Obviously there are merits of going the recon route - but it depends on your outlook. The car could be written off next month, or another major repair could arise!
Will