All of those tweeters and 6.5 and 7'' drivers are all for car applications?! Some I've never heard of for cars! All look good though! So this would be the way to go huh?!
Most of them are 8 ohm speakers, not your "standard" 4 ohm speakers that are found in all of "car audio" speaker sets. However if you will do reading on 8 ohm drivers you will find that their is basically nothing wrong with using an 8 ohm home speaker in a car application. You have to have a bit more power able to power the 8 ohm speakers because they are a bit more hungry, however that usually is the least of your issues.
You can use any of those speakers on that website in car applications, as said the main difference is most of them are typically used in home bookshelf systems etc.
Are you looking to run a 2 way active front or 3 way active front? I would recommend a 2 way active if you have never ran active before.
Will you be using a headunit built in with active capabilities, or will you be using a separate device?
You will find that if you are looking to spend $500 on a pair of mids and tweets, your install must be perfect, or near perfect. First you should completely sound deaden your doors. If you dont know what I am talking about, right there you have alot more to read up on before you buy $500 mids and tweeters.
However if you already know all of that, Depending on what mid you can fit, the Usher 8945P is a BEEFY *** mid, Weighs in at about 6-6.5lbs, the magnet is Huge, Also the Seas Excel w18 is an extremely nice woofer.