1. Don't lower your car too much!!! (mustangs limit for good handeling is about 1.5-1.75") Why? Because aftermarket springs are stiffer because they have less travel available to soak up a bump. Why this is bad... small bumps in the road make the wheel bounce, reducing tire contact with the road, thus, less traction.
2. Shocks. Lower your car, you HAVE TO HAVE good shocks. I would suggest the KYB AGX's because they are cheap, rugged, and adjustable. They will allow you to tune out some of your rear chassis leanand understeer as well. (note, rear pinion snubber needs to be trimmed 1" on a lowered car)
3.Wheels. Wider is better. 17x9 sounds fine unless your running over 400RWHP. A place called OE wheels on ebay sells nice rims. They are here in my town and have a nice product. Its like $400 for a full set of rims. check them out.
4. Tires. A good tire on a narrow rim, gives the same handeling as a ****ty tire on a wide rim. So do the rims first and save $$ in the long run. Kuhmo ecsta MX's are about the best I have found for the front to use for daily driving and auto x. This is just my opnion, but then again, I have had over 20 different brands of tires on my car and race every weekend. they are cheap too. Run a 275/45/17 up front on the 9" rims.
5. rear tire/rim. For a few extra $$, you can get the 10.5" wide rear rims. They look sexy, and are great. Do them if you can. also, Sumutimo makes a 315/35/17 tire that is only $95. They grip great for cars under 450RWHP and look great too.
6. Sway bars. You need an adjustable rear sway bar. Either an add on one, or stiffer one. I prefer add on, because it is easier to fine tune.
7. Bushings. Dont bother with
lower control arms, not worth the $$. But do invest in some poly bushings for them and your steering rack. much quicker turn in and less understeer. More road noise though. as for the rear, get some good
lower control arms and watts link or something simmilar. When you corner, the rear axle moves side to side up to 2 inches!!!! this will eliminate this. DO NOT get upper control bushings!!! touching anything up there will hurt handeling!!! Unless your looking to only go straight.
What I would do: get the front coilovers. get taller and softer rear springs. (cut 1 coil of stock springs) this will drop car 1-1.5". more than an inch of drop in the rear kills weight transfer making it impossible to hook at launch. Then start at #1 up top and work your way down. By #4 you will have a very fun car. By #6 you can get a .90G sideforce.