changing the plugs isnt that bad, the stealer is always a rip off. you just have to have a few different lengths of extensions for the ones in the back. the regular 3800 likes delco platinums or e3. look on rock auto. i would do that first, clear the codes and see what happens. then if it comes back it might be o2 but since the cat is original its likely the issue. as long as there are no exhaust leaks. you have to get a good oem type cat to maintain emissions compliance in your state. rockauto probably has one of those too. if you dont want to do it yourself call around to a good rated local owned shop. they have much better prices and are much less likely to rip you off.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock