Matt, a bit of advice... going up tech trees is faster if you concentrate on one tree at a time. Then again, this would depend on your play style, I guess. There are some players out there who have tons of Tier 4-6s from different trees, while others specialize in one tree and go all the way to Tier 9-10. If you want to reach the high tiers faster and without spending an insane amount of money (I know a few on my clan who "limit" their spending to £30-40 per month), I recommend you specialize.
All nationalities have their pros and cons. Take the SPGs for instance. Russians have better arcs (can hit behind rocks better) and thus have a "rounder" target area, Germans pack more punch... etc. I know this because I have both Russian and German SPGs. Having said that, some believe there is a lot of bias towards the Russian tank lines, but I don't know much about the Russian or German tank line to tell (I play mostly TDs and SPGs).
Getting kills gives you bragging rights and there's a medal or two for that I guess... Reaper comes to mind. However, I am happy enough to "contribute to the team" in whatever way possible. Heck, I've even spotted the entire enemy army once in my A-20, never fired a shot, died within 3 minutes, but still got a neat chunk of XP for my "detected" tanks.