Ok I'm gonna put my 0.02 cents in here...
While you're smart for wanting to go with new shocks when you do your springs, there is some flaw in your judgment.
Firstly DO NOT buy Dropzone anything... Why get Steeda springs and then ruin it by going cheap on shocks(<--- spend money here.)
There are some tried and true way of lowering mustangs, here is a good example:
For shocks the best 2 "budget" ways to get a good ride is either Tockico Blue(HP series) or the KYB GR2/Gas Adjust combo.
These will be able to provide sufficient damping for a lowered car. If you want to go 1.5" or more then you'll want to look for Fox Body fitment struts/shocks because they are shorter and will not overextend under extreme lowering...
The other option is to buy it nice(or buy it twice) so that you have good components that work with a wide variety of lowering springs and will last a long time.
Like mentioned Koni is a great brand, but theyre a tad expensive... Something along the line of Tockico Illumina or more preferably Bilstein HD(what I use) is pretty much the best street choice out there.
While Steeda springs are very good pieces, in the stance and springs rate standpoint you would be better served by H&R's offering...
The Super Sport springs will go really low and ride great(only if you buy good shocks though.)
The Race springs are of a much harder spring rate but are still progressive(you don't want a fixed rate spring on a street car for the most part...)
Lastly I will leave you with this:
GET Caster/Camber Plates. I would say that anytime you lower a mustang you should get CC plates to get the alignment PERFECT and not worry about running out of adjustment on the stock hardware.
I am glad to answer any more questions you may have.
Furthermore, if you need this suff installed I can do it for a much better price than any shop will quote you. PM me