[LONGHAIR]

It is so much easier to keep this straight if it's all in one thread. I didn't see this one until now.
Used gears (IMHO) are a loser in the cost/benefit analysis. The extra (read frustrating) time is just not worth it in the long run. You wrestle with it for a long time, get it somewhat acceptable (as good as your patience will allow), go drive it, and one of three things happen.
You win the lottery
It works, but is noisy and you live with it until it fails
It works, but is noisy and you can't live with it.

The 2nd or 3rd are more likely, so now you are doing this work all over again. This time with new gears and new bearings because of the garbage that went through them while making that whining noise.

I did this far a living for almost 11 years, done hundreds of them. Used ring and pinion is a rear axle almost never work, unless you can take them out of the original axle yourself. Even that assumes they were ok in it? It's kind of a self-defeating thing. By the time you test drive a sketchy set-up, you have likely ruined it.
I replaced a set in the rear of a truck where the guy did it himself at home. He drove it less than 20 miles before destroying them.
On a front axle, which sees far less use (and almost all of that it at low speed), you can "get by" with a lot more.