yea dude, stay away from the full-on mx'ers. its overkill unless you like jumping it and ripping full bore all the time. i'm currently in the market for a wr450. its basically the yz450f, but more trail-oriented. different flywheel weights so it has more low-end power, i believe they have different cams too. anyways, i rode one back to back with an rmz450, and while the rmz was a more "badass" bike, the wr was everything i could want/need with the addition of electric start!! dont know about you, but when i first started riding dirt i HATED kick starting a bike. just my thoughts, good luck dude.
oh, and as is the case with most toys like this, maintenance is the key to longevity. ask if the bike gets serviced after every ride, or every couple of rides. oil changes are hugely important to a 4-stroker.