So "handling" is an interesting topic- and you need to answer one question before your proceed to the mods.
Do you want to set a fast time, or do you want the car to feel really good, or do you want it to do both?
If all you want to do is run D Stock and set a good time, go get the stickiest tires you can afford, get a stiff front bar, and go to town. If you want the car to feel good, wait on the tires and do dampers and stuff first.
For your purchases- I would direct you to Vorshlag LLC
They placed 4th nationally in ESP in autox in 2012, and third in NASA TT3 this year, so to say that they know how to make a Mustang handle is a gross understatement.
Some points of advice/things I have learned-
-Get a Torsen.
-Figure out what your goals are, then buy the correct dampers for those goals. I wasted a lot of time and money going through three different spring/shock combos before I settled on what I have now.
-Chinese shocks are not going to last, and are not going to really perform. Buy USA or European made if you want to "do it right"
-You need good camber plates
-You need sway bars
-Get 18x10 wheels that weigh 20# or less
My "list" for a DD that autocrosses or goes to the track occasionally
- Bilstein non-adjustable shocks & struts
- Vorshlag camber plates
- Lowering springs of your choice- don't go more than ~1.5" drop
(for the above three, check this out...Vorshlag-Bilstein StreetPro Monotube Suspension Kit (S197 Mustang) - Vorshlag LLC
) read the whole thing for a good treatise on why lots of shocks out there are doing it wrong
- Sway bars (I like and use Whiteline's bars. Great endlinks, lots of adjustability)
- LCA relo brackets
- Adjustable LCA's (or get the Boss 302R arms, they give you a good pinion angle when used with a lowered car and the Ford Racing relo brackets)
- shifter bracket and whiteline bushing insert
As you get more radical, you'd get into coilovers, different diffs, seats, harnesses, UCA, etc.
Let me know if you've got autox setup questions, I've tried lots of different things and have some real data. Alternately, call Vorshlag and tell them what you need. They'll be more than happy to shoot the sh!t about various setups and give you different ideas.
Good luck, and have fun!
PS- DO NOT get subframe connectors. They are absolutely not needed. The S197 chassis is torsionally stiffer than a Gallardo