If all you are looking for is keyless entry, here's a thread I started on putting a keyless entry in a W124 for less than $100.
I did it! Keyless entry for your 124 made easy (and less than $100)!
Did the same thing a year ago for my '94 C280. Took less than an hour. I also posted a thread on it, as there are differences in the installation:
C280 Keyless entry from "the keyless entry guy"
'57 190SL (toy)
'08 S5500 (mine)
'09 CLK550 (wife's)
'06 SLK350 (daughter's)
'11 GLK350 (daughter's)
'03 CLK310 (spare)