What next?

[thstone]

For future reference, misfires are the most important issue in the above list and should be promptly diagnosed and repaired as continuing to drive an engine with consistent misfires can cause damage. O2 sensors and high idle are emission related and are not as critical.

Good to hear that the codes haven't returned. I suggest buying lottery tickets this week as it seems that you are on a good luck streak!

[Perfectlap]

your coil packs are probably on their last legs by now too. That's the only time I ever had a misfire CEL, I put it off for a bit too long.
Before you pour heaps of money into this car have a compression and leakdown test done. A car with no history is probably one that hasn't had much maintenance.

As far as most economical that might save your engine, order the latest version coolant cap. Don't flush the coolant without a new one. Air will get in the system and split your old brittle plastic tank...small leak... turns into a dump of all your coolant on the street.