[KiltedBanshees93GT]

As to your first question, a k&n (or in theory any of the reusable filters out there) uses an oil coated cotton gauze for an element, instead of a paper type element. Net result, more airflow, which = more hp, and you can reuse it. Buy a 14 dollar cleaning kit, and IIRC k&n guarentees them for like a million miles. In other words, buy the filter and occasionally (every few years) cleaner, and never have to replace the air filter. Its pretty much the first mod done to every 'Stang out there.
For simple stuff right now, I would get her tuned up at a shop, and buy the manual for the car, and read it. front to back and vice versa.
As noted before, if you can, and if not have someone do it for you, you can bump the timing a few degrees, start thinking about a good exhaust setup, basically it depends on what your comfortable doing or having done and money.

J