there not hard to change just need to be easy on the inside bolt since you can not get them from ford at all. As for buying a used arm save your money un less there vary new. Because the Inside bushing goes and thats what needs to be changed out.
If the bushing is no longer avail from ford is it possible that there is an aftermarket bushing in a poly. or something like that? What do you do if you find you need new ones???
Not sure about the upper arms there Merc. The lower ones are the aluminum ones and ya need a spring perch to level your springs. With the perch and the knuckle attached they are lighter then the LCA from the bird alone.