At 350k you got full use out of the cat. Factory cats are the way to go, but there are aftermarkets. They just don’t last as long.

It won’t be long and the other side will soon follow. Just how these things go.

If I read it right, its a no crank, no start when you turn the key. If that’s the case I would suspect the starter is going out. But also check your battery connections.

First question is there Any oil on the dipstick?

Second, is the belt on the engine pulleys?

I would not put coilovers on your 1990 corolla. OEM style struts will suffice, ride much better, and won’t destroy your other suspension components. Quick struts are the way to go. You’ll pay more for the parts but it is a much quicker and thorough install as you will be replacing the strut tops, spring, bumpstops, and strut all in one go.