Sounds like you will be right at home here - we all have a love affair with our luminas. when was the last time your lumie had a full tune-up? Plugs, wires, pcv check, oil, air, & fuel filters, plus some GM top engine cleaner or some seafoam to remove the carbon build up from your intake and your valves.
The oil leak may not be his fault. When GM decided to stop using distributors on our cars and go with coil packs, they just put a cover and an o-ring where the distributor would be, and it is notorious for leaking on our cars. Problem is to repair it correctly, the heads must come off (lets hope thats not your problem). There is a quick fix but some folks advise against it.
As far as the muffler, shop around at some other places, you can DEFINATELY get a muffler cheaper than $150.00. A clogged catalytic converter can cause performance issues too.
Also, at your next tune-up make sure they clean out your idle air control passage.
Welcome to the madness.
Oh, and you mentioned your tranny fluid is full, but when was the last time the filter was changed and fresh fluid was added? (just a thought).