IMHO, unless you are an advanced racer, having a newer bike isn't going to make a whole lot of difference in your speed or fun factor... I was doing research on used yz250's, and it seems like the 2003&2004 model yz250's were the cream of the crop for those years... Not sure about 2002... I ended up buying a 2003 kx250 because I got a great deal on it, even though it was generally pulling up the rear in comparison tests... Again, imho, my skills are not up there enough to where the few tenths of a second advantage the other 2003 bikes (or newer ones) might offer me will make much difference...
2-stroke engines are so easy to adjust the powerband on, I wouldn't not buy a yz just because someone didn't like the power band on their 2k model. You can do a lot with jetting, adding flywheel weight, etc to modify the powerband to your liking... I had a 99 model and liked the power band just fine...
Even IFF an 06 yz250 would increase your lap times by 1-2 seconds which is HUGE for a serious racer, would that make it worth an extra $3k+ for you??