I cant forsee you needing to spend any money on a carrier from a used axle.
There is nothing to go wrong with them. You may need to put some new bearings on it.
I just mentioned that because you could pick up a whole axle for the price of just buying a new carrier. Do some research on which carrier you need for you gear ratio, and take a look at the axle before you buy it...
If it was me, id buy a whole nother' axle or look for someone with a carrier laying around
I was not suggesting using the "new" used axle. Pull the carrier and scrap the rest, or keep parts for spares... You would have less into a carrier than buying the new questionable quality one. (I do not think you'd have any problems with the motive gear carrier however.) I was addressing the last part of your post the least costly way to do the swap..