Yes, old lthes do wear out! Especially since most of your work will be within 12" of the chuck, that portion will be worn down.
You will pay more for a used lathe (A GOOD ONE) THAN A GRIZZLY WILL COST YOU AND YOU'LL HAVE THE WEAR THAT HAS ALREADY TAKEN AWAY THE PRECISION YOU NEED FOR GUN WORK.
I fought the very battle you are when I purchased my Grizzly.
After WWII we left these countries with a "military standard" set of measurements, which they, in turn, adopted. Everything they build (in machines) is made to a "mil-std."
With the advent of CNC production and "mil-std" they produce as accurate a machine as is possible!
I used to have a Rockwell turret mill and while it was nice for pistol work it lacked the capacity and features of my new Grizzly. Don't think I've got money - I don't. I sold off three guns to buy my mill and had to sacrifice a beautiful Hawken muzzle loader in the process I'd built years ago!
Go with the new Grizzly - you will not be sorry!