Get the 500AF, trust me. There's no better dirt bike in the world, 2 stroke or 4 stroke, big or little, regardless of color, creed or sexual orientation. Suggestions:
RG3 triple clamp to reduce vibration;
ERT chain;
At least 49T rear sprocket (48T stocker = 95 MPH);
Good tires;
Universal or ProTaper bar (another vibration tactic);
No flywheel weight, just don't turn the throttle all the way if you don't want the tire to spin;
Billet case covers (no funtional reason but just look too cool to pass up).
Don't worry about keeping the bike more than a year - it'll take longer than that to wipe the grin off your face. Also, you won't give a hoot about the power valve or lack thereof - the powerband is about the size of the Hoover Dam as it is.