How much can I lower it?

I am about to buy some lowering springs and before I did I wanted to ask how much I can lower it without needing to upgrade the rest of my suspension. I thougth I read years back that you could lower it 1.5 inches, but anymore than that and you would have to do other suspension mods. Is that true? Also, I thought I've read it on here before, but wanted to know for sure if I needed caster camber plates. I think someone said you dont but the first time you get an ailignment you will have paid for them because the alignment is that much more. Lastly, if anyone has the Eibach prokit or sportlines let me know how you like them. I guess more importantly the sportlines. My 90 5.0 has the prokit and I like it just fine and also scrape a decent amount and I wanted to know how bad the sportlines were with clearance. I'm going to be buying them tomorrow so any and all help would be great. Thanks.
 
You can usually lower it up to 1.5 inches without needing c/c plates. Anymore than that, and they're usually needed to get the proper alignment back out of the front tires. There are no other suspension mods needed when lowering a stang though. My cardomain site has some pics of my stang with the pro-kit on it.