hi Sky,
I had similiar problems as you (still do) OK, here goes, but it will be long, you have been warned!!!
When I first purchased my S14 (as well as the first one I had) both had leaky oil gaskets. The first one, I made the dealer fix it. The second one I purchased was from my work (a GM dealer, got it in the wholesale lot at wholesale price!!

) So I told my service manager to fix the gasket. He replied its just normal wear and tear, its not really leaking, its sweating. His analogy, and I quote, "A leaky oil pan is like a leak dik, even if its a little lubed on the end, it still works!" end quote. Real funny guy, still haven't repaired it to this day (about 5 months now), and my oil is fine, no drips or puddles, nothing. I would take the route you chose; slap everything back together, clean the undercarriage, and keep a very close eye on it. If the leak seems to be getting worse, than fix it. If its just sweating oil, I'm sure the car can handle it. Don't have any recomendations on gaskets, but an OE replacement should be fine.
Just noticed my diff seems to be leaking, I'm gonna get it flushed and cleaned, then take it from there.
Rough idle is a problem I had before, and when I do a cold start up, my revs fluctuate up and down, and will actually die (stall) if i don't give it gas. Also taking off in 1st, I give her a little throttle or else there is a slight hesitation, but aside from that, all is well. But it wasn't before.....
My car had a serious loss of power between 3300rpm's -4000, then the power would come back. Everyone said its a vacuum leak,checked all pipes, everything was good. Played with my MAF (shook it around) a bit, and at times it would help, but then go back. Replaced my fuel filter with a 300zx TT replacement, helped throttle response, still had the loss of power. Know what it was?
A stickt EGR valve! Real easy fix, just get a can of ronson's lube, and grease the bad boy up! It looks like the throttle cable (for lack of better word) that sits on your engine. Y'know, you pull on them while the car is running, and it revs the motor? Anyway, looks like that, but sits vertically on the passengers side. There is a cable attached to a valve, and it should move freely. Mine was completely seized, I lubed her up, worked her in a bit, took her for a boot, power was regained!! And it cost me five dollars!! I love those fixes!!!
Oh, I also tightened my throttle cables to improve throttle response. Real easy, 2 nuts on either side, loosen and tighten accordingly. You will see 2 throttle cable, when facing the engine, the one on the left is for cruise control, the one on the right is for idle. Becareful though, you can mess ish up bad, just play around and fine tune, and please, take your time!!
I also cleaned my throttle body, helped a bit. The vibration you are feeling may be due to a bummed out rim, thats what I had when I put a warped rim on my ride. Quick fix? Rotate the front wheels to the back. May not solve the problem, but it will take the vibration away from your steering wheel (hopefully).
Don't go to Maaco for a paint job, they did a nightmare job on my boy's supra, and is no where near the quality of the paint job I got for half the price! Ask around, a buddy who owns a car rental store gave me my shop, great price, excellent work, he just had my car for a while, but did all the door jams, under trims, lights, ect. ect.
Gunmetal rims are hott!!! I need a set BAD!!!
I purchased a universal bruellen muffler and got custom piping from the resonator muffler back (want to engine swap one day, see no need in spending big bucks on a exhaust I'm trashing in the future anyway, true?).
muffler - $200 Canadian (I think it was stolen, right place at the right time situation, usually retails $500+)
piping - $145
great exhaust, sounds nice, very low, deep rumble. I did free up some ponies, but its no neck breaking experience! I say go the cheapo route until you do an engine swap or have some serious work done to your KA to support the crazy cash for a bolt on cat back system that nets you 4 HP, for half the cost of a nitrous kit, right? I would take your friend up on the free exhaust offer, and save your cash for something with a better return, know what i mean? And if it truly sounds ricer and is bothering you, then pass it along the line, I'm sure someone will take a free exhaust!
These guys seem genuine about their offer to help you out, I'd jump on the offer asap, nothing like free advice!
hopefully my ramblings have helped, if I can think of anything else, I'll let you know
take care
PauL
whoa, I'm done.....