Ahhhh... a classic Grady post. I haven't seen one of those in a good while. Well done, sir!
Nick- this process is all about taking it slow, doing your research, drawing out a detailed plan on what goals you want for your car. Would you rather your car look good, or go fast? 99% of younger Mustang owners, like myself, do not have money for both- you have to make up your mind. Also think about what kind of offroad racing you might have an interest in- autocross/ road racing or drag racing. Go to your local tracks and see what you like. Lastly, learn from the old hands and avoid any mistakes they might have made along their quest for horsepower. Do all these things before you reach lighten your wallet!
My personal mistakes with my car were a result of little to no research and getting an itchy trigger finger when I saw "add to cart." I still kick myself for the amount of money I've spent on parts that I really did not need. Also, it doesn't hurt to buy used parts in certain cases (used intakes, exhaust, short throw shifters, CAI's). In my opinion, things like gears and cams should be bought new.
This is my opinion on parts you really don't need to be wasting your money on in the beginning steps of adding modifications.
-K&N FIPK- sure, the filter has the capacity to suck in a bunch of air, but it's sucking hot air in right from the engine and radiator! It is also WAY overpriced. As stated in an earlier post, simply remove the air silencer and get a stock style K&N filter. I made the mistake of buying this, and I still regret it.
-Subframe connectors- I have them, but I got lucky with that purchase since I ended up autocrossing my car. But for the most part, you don't need them- they stiffen up a daily driven car
way too much. The creaks and groans in my car drive me crazy...
-Headers- your car already has them from the factory- why waste money on aftermarket headers? They will hold up fine until you do a full H/C/I upgrade. Theyre not cost effective, kind of an annoyance to install, and wont give you much gain on a stock motor.
Now, underdrive pullies actually DO make more than just a SOTP difference. Now that I have them, my car spins halfway through 2nd gear instead of just chirping 2nd (that's with 3.73 gears).
If I could start all over, I would plan on: 3.73s, short throw shifter to compliment the 3.73s, underdrive pullies, offroad midpipe, upper/lower intake, small cam, bump timing to 14°, cheap CAI.
If I had extra spending money after that, I would get a catback.
Car Nut explained the maintenance bit pretty good. You should do those things first to make sure the car is ready for mods.
If you want a comparison between the H pipe and X pipe, let me know, I can post links. I used to have an offroad H, now ive got an offroad X.
OK IM DONE- Ive typed
WAY more than I thought I would have.

Many of us here have been in your situation before. I just wanted to give you a heads up for what's to come! Good luck man!
-Wade