As pointed out by the previous poster, changing tranny fluid on a 722.3 transmission is pretty easy. There have been many discussions on this subject here.
If you elect to do-it-yourself, you'll need to elevate the car in some way to gain access to things as crawl space on a 124(or 126 for that matter) is somewhat limited.
I use heavy-duty metal ramps and drive the car up on them just enough to provide the needed space. I also block the rear wheels. Once this have been done, you have clear access to the converter(you drain it too) and the main pan that's a little further back. Filter is inside main pan. 5mm allen head removes both the converter and main pan drain plug. Do not over-tighten main pan bolts and use use pan gasket. Replace small washers that are on drain plugs.
The 103 motor/transmission are fairly easy to do basic maint. on. Do-it-yourself and know that it REALLY got done.