Thanks for the missing information. The 2000 Celica was sold in a GT model; which had a 1.8 liter 1ZZ-FE engine; and in a GTS model; which had a 1.8 liter 2ZZ-GE engine. These 2 engines used different spark plugs. The plugs you listed are intended for the GT with the 1ZZ-FE engine. There should be an emission label under the hood; which lists the engine model. There should also be a sticker on the valve cover with the engine model symbol on it.
If the exhaust system has been modified in any way; such as installing an aftermarket muffler, removing or bypassing the catalytic converter; or if there are holes in the pipes or the pipe has been shortened; the engine will run too rich or too lean. It will not be possible to make the engine run right until the exhaust system is complete and undamaged; with all original components.
The oil under the coils is coming from either a leaking valve cover gasket, or leaking spark plug tube seals. This should be attended to. Oil on the outside of the plugs will also cause misfiring.
I would recommend replacing the fuel filter, the PCV valve, and thoroughly cleaning the interior of the mass airflow sensor, the throttle body, and the idle air control valve. Replace the air filter element if it does not allow sunlight to pass easily through it. Make sure that all the vacuum hoses are conected properly, and they are not loose or damaged. Replace any damaged hoses. Make sure the intake manifold bolts are tight and none are missing.
Stalling can also be caused by a defective camshaft position sensor, or a defective crankshaft position sensor; as well as by bad oxygen sensors. The GT and GTS engines use different oxygen sensors and different cam and crank sensors. There are 2 oxygen sensors on the car; and I recommend replacing them both. Replace the cam and crank sensors if the stalling continues.
The catalytic converter can plug up with this many miles on the engine(s). A plugged converter will make the engine lose a huge amount of power. A good test of the converter; if it has not been removed and the exhaust system is intact, is to stand behind the end of the tailpipe while someone sits in the driver's seat with the parking brake on and the engine idling. Hold your hand behind the end of the pipe and see how far away from the pipe you can hold your hand and still feel the exhaust pulses. Then have the person in the car step hard on the accelerator. There should be a blast of sound and pressure from the pipe. You shpould be able to move your hand twice or three times as far from the end of the pipe, and still feel the exhaust pulses, when the accelerator is pressed hard. But if there is little noticeable change in the exhaust sound or pressure when the accelerator is pressed hard; the catalytic converter is probably plugged up.
www.rockauto.com or www.autopartswarehouse.com have the best selection and prices on catalytic converters.