Engine problem - misfire. Electrcal or mechanical? How to test most efficiently?

I need help diagnosing an engine problem on a 1997 Buick Skylark with 2.4L 4-cyl engine. I had a sporadic misfire for a month or two. Suddenly it got worse all at once, the engine ran really rough, and the car stalled out. After I tried to drive a little further, it stalled again and I parked it. While I was pulling over to park, the driver behind me stopped and told me that the underside of my car was glowing hot. By the time I got out, the glowing had stopped, so I didn’t see just where it had been (catalytic converter, perhaps?). I had the car towed home. Now, when I disconnect the electronic ignition, the starter turns over and it turns the engine over nicely (doesn’t “catch”, of course). But when I reconnect the ignition, the engine strains and makes a horrible, violent thud. Before the car died, my OBD reader showed 0300 - random misfire, and 0440 - EVAP system malfunction; one time, the day before the car died, it also indicated a crankshaft position sensor problem, but that did not repeat.

I pulled the plugs out. The outer two (1 & 4) are light brown, no corrosion, buildup, etc. The two middle plugs are black with just a touch of oil, not really fouled. They hadn’t been replaced in years, but I don’t know how many years. Changing the plugs did not help. There’s no apparent water in the oil or oil in the antifreeze, though cooling system tends to need topping pretty often. Car has 140k miles.

I’ll begin to test systems one by one, but can anyone offer some guidance as to what I should test first, and what my problem is most likely to be? This car has an all-electronic ignition (no distributor), with coil packs directly over the plugs, and fuel injection. Thanks for any suggestions.

One more question: Have you considered initiating a payment option through PayPal or some other well-known company? I am cautious about giving out my credit card info to companies that I don't know well. Thanks again.