The kit should come with the horns you need. If not, some horns will come with the servo that you could use, but its easier to use the ones with the kit.
The springs that come with the kit are just fine, you can get 15+ feet of air without bottoming out with the stock springs.
Oh, and another charger you could look at is the charger that comes with the Revo...thats a much better charger than the other one I linked you to.
http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXMAD6&P=ML It also chargers 7.2v stick packs in case you want to use roto start.