Mettler is a perfectly good thread and the 50 size is what's recommended for sewing on a machine. Hand quilting is size 40. My LA thread is size 40. 40 is a thicker thread. So 50 is perfect for a DSM.
As far as using old thread, I do it all the time. I do remove the outer layer of thread from the spool and do a breakage test on it. I've had new thread break more easily than some of the very old thread on wooden spools so it really depends on the amount of time it's been exposed to the air and sunlight.
So, don't worry. That Mettler is just fine.