http://www.americanmuscle.com/99-04-mustang-springs.html I like the stance the H&R Super Sports give the car but if I were you I'd go through the photo galleries on American Muscle's site to see which springs would suit you best.
Eibach Prokit is good, if you just put springs and isolators it will be lower than if you add the struts. i would suggest the struts tho, my car tends to bottom out a decent amount
idk i wouldnt go more then one , my car sits really low with 1.5 drop sometimes i hit the ground so gotta fix that but personally 1 inch maybe 2 no more then that
out of those 2, go with the eibachs- H&R's can be a bit stiff, plus youll have to either run no iso in the front fo get the full drop(no V8 weight), or poly in tehr ear to correct the reverse rake.
about another .25". but it can really depend on other variables- others will be lower with isos because their factory isos were more beat down, or less because they run poly iso's.