I would not buy the kit. swaybars are not nessesary. there are better springs I think. search for threads about sagging
eibach springs. then search for thread about sagging h&r springs. you will find more people complaining about the eibachs.
I mean everbody gives a thumbs up to what they have. I did not buy eibach based on the number of people posting long term issues with there stuff.
for the money of that kit you could buy a better setup.
every mustang needs subframes and
LCA's to handle well at minimum. with the kind of money that system cost you could have those.
shocks and struts make it handle better but your car will still handle poorly without something to reduce flex and locate the rear axel better.
for the money of that eibach system you could piece together a kit that would outhandle it going away with just as good if not better street manners.