$63 is a good price. Fast lane sells them for $70+ but you can find them for less at e-bay. Not difficult to replace but, are you sure it is the cap that is cracked or the just the black cover?
Oil leaking from the upper timing chain cover is NOT rare on older 124's. You may be able to DIY.
You can get the official MB CD from Mercedes for about $100 or try e-bay for a "copy" for a lot less money.
Use a search on these topics on the right upper corner on this page. They have been discussed before!