safari engine codes, runs rough

Hello all,
I have a 1997 Safari AWD 146,000 miles, with the engine light on. I have done a code check and got P300 and P420.
The engine runs rough at idle, and gas mileage is terrible. Seems to run OK at high speed.
Repair shops wants $500 for new plugs, wires, cap, rotor....
Auto part store guy told me it would be my O2 sensors....
$150 apeice x 2 plus labor..
Could end up at $1,000.00 for both...HELP!
Any thoughts would be greatly appreciated!
Thanks,
Joe

P0300 is a misfire code...could be caused by an ignition fault or a fuel injector problem.
P0420 is a code for a degraded catalytic converter..most likely caused by the misfire..
First thing before any parts are replaced is to properly diagnos your rough running condition to confirm which componant is causing the SES light to come on.
If you have not had the recommended maintenance (IE: plugs, cap rotor) at 100k, I would definatley recommend that a "tune-up" be done.... (only after the complaint has been resolved)

After misfire problem is solved will the p420 fault clear up on its own.
would there be sensor damage or cat damage.

99 safari with blown coil.
missfire and engine flooding.

I'm kind of hoping this will clear itself up.

thank.
Marcus