You can probably get the car into the mid-13s with sticky tires and a set of 3.73s in the back, assuming a manual gearbox and a decent driver.
Re Catback exhausts: Check out
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As Night Shifter notes, exhaust note and volume is very subjective. They're all going to give similar power gains so just listen to some clips and decide for yourself. See if there's a local get-together of Stangs that you could lurk around to get a "real world" feel for the sounds. Then you can ask the guy about the brand, the drone, power etc.
Re CAI: As long as you're actually taking cold air from the fenderwell and not just sucking hot underhood air make is not that important. I had a JLT pre-blower and liked that it put the MAF in the fenderwell, giving it the air a straight-shot through it from the filter. I'm not too keen on CAIs that put a 90-degree bend just before the MAF, especially if there's no screen or flow-straightener at the entrance to the MAF.
Re Pulleys: A few -- maybe 4-5 -- HP at most.
Re
Suspension: Again, this is pretty subjective. Some people want near-OE ride quality while others want "slammed" or road-course-worthy or drag-strip-worthy. I went with a Bullitt kit from FRPP on mine and am very happy with it but it's not at all aggressive nor does it lower the car more than about 3/4-in. I can recommend this kit if you're after tidied-up, OE-like ride and handling: the 2001 Bullitt was equipped with this from the factory so you know it's a pretty well sorted combo.
But again, everyone's tastes are different. Whatever you get, I'd buy it as a complete set if possible: struts, springs and sway bars all engineered to work together.