Yes, the how old question is quite valid.
I have one from the early 1990s and have had no problems using G2 barrels on it. If you have an older one that doesn't have the "easy open" modification, T/C can do it for you.
As far as what barrels you should buy? You say you have 222 and 357 barrels but don't mention the length or what kind of sights they have. In my opinion, with the excellent accuracy the Contender is capable of, it's well worth scoping the barrels. A 357 Magnum barrel should have quite adequate accuracy out to 50 yards but may not have enough oomph for good 100 yard accuracy. I took the above Black Wildebeest with a scoped Contender with a 10 inch 357 Herrett barrel that's capable of 1.5 inch 3 shot groups off the bench at 100 yards.
If you want a varmint rig, your 222 barrel would be fine. I have a 222 heavy rifle barrel for mine and have taken every thing from rabbits to deer with it. A 30/30 barrel would be even better for deer. If you want an elephant stopper, go the the 375 JDJ (yea, I have one of those as well).