The first link is the 8 ohm version, the second link is the 4 ohm. The 4 ohm doesn't have the shield, so that's the one you want to use.
The Dayton RS series is some of the best bang for the buck speakers out there. However, with the low cost comes some compromises. They have some pretty bad cone breakup. The RS180 (7") breakup starts at about 3 khz. That's pretty low, and takes some considerable crossover work to deal with. So, the driver is really only useable to about 2 khz, with at minimum 18 db slope. Also, finding a compact dome tweeter that can handle 2 khz isn't easy. The Seas Neo will do it, and is inexpensive at about $30.00 a tweeter, but you're pretty much limited to that. Not that it's bad. That setup will knock your socks off, especially for the price. You just don't have any choice, unless you go much more expensive, or use a larger format tweeter.
All that being said, having a properly designed 18 or 24db passive crossover built for you won't be cheap. Going active won't be cheap either. Getting a boxed component set is easiest, but the results won't be as good.
You don't need a new head unit to do active. You do however need an external active crossover, and an extra 2 channels of amplification. The extra channels don't need to be high power, as it's usually just used for the tweeter, and they don't need much power. 50 x 2 is plenty for a tweeter.
Here's an example of a decent outboard active crossover.
http://cgi.ebay.com/ebaymotors/USED-Coustic-XM-5e-Mobile-Electronic-Crossover_W0QQcmdZViewItemQQcategoryZ38642QQitemZ200087051061QQrdZ1QQsspagenameZWDVW
I actually own one of those, and it works very well.
If you're looking for more information on active tuning, and DIY speakers in general (especially the RS180), check out
www.diymobileaudio.com