98 Cavalier Z24 - simple fix, or haul it to the car graveyard?

My husband's 98 Cavalier Z24 has somewhere in the neighborhood of 150K miles on it. Recently, the engine light started coming on intermittently and then, before we could get in to the mechanic, sputtering a bit. He said the other night, the tach started "flipping out" (I would assume this to mean jiggling) when he was on a major highway - without the engine missing or anything (makes no sense to me either). He also told me that when he slows down, it sputters a lot under 20mph and also wants to kill when fully stopped. When he started it up the other day, the exhaust reeked of gas (it was also a little bluish, but the car hadn't been driven for two days and that's normal for that car). Opinions? I'm thinking either oxygen sensor or catalytic converter. Or, possibly a bad tank of gas.

As the frame is rotting out, I do NOT want to spend a whole lot of money for a fix on this. I'd rather go drop 7K on a used car. I'm not even sure they'll let us fix it, I had a hard time getting it away from the mechanic the last time (because of the safety issue). However, I do need my car back. Of course, he's starting full-time (but yay) work the last week of September, and this is only going to get worse if we can't fix it cheap....we can't make it work with one car for long.

Let me know your thoughts and if you need any more info, let me know and I'll tell you what I know. Thanks!