Hey there, nice to meet ya. Always cool to hear someone like you so excited about getting a new bike
Gives me butterfly's.
My input would be to always get the newest year you can afford that is in moderate shape. Doesn't mean that it will necessarily be in great shape because even a 2002 KX5 might have been run hard, and I mean hard in the desert of Nevada or California and not be worth a crap. (suspension, all bearings on wheels/swingarm, frame tweaks, bad radiators...)
Guess it is up to what type of riding you are going to do with it right away?? Easy desert rides or WOT baja racing?? Running WOT at the dunes?? Cruising powerline roads or running pikes peak??
One thing is for sure... unless it is new or just renovated like new... stuff will be wore out and broke on the KX5 you get. I bought a 2000 KX5 from the original owner in his mid 50's last year for $2300 that was set up for desert riding (steahly flywheel weight, FMF Gnarley, V-Force 2 Reeds, Pro-Taper Bars, GPR damper with triple clamp, bark busters, and oversized desert tank) that had very few hours on it but he dumped it over alot. He couldn't kick start it after he fell over so he bought a KTM 450 electric start and garaged the bike for a year or so. After I got it home and started tearing it down I found all the damaged items that are inevitably going to be there on these bikes. But I knew that going in and $$ with time would be needed. Since I was setting this bike up exclusively for dune riding I would sell all the items above on eBay so I could turn them into the parts and motor work for "Furious".
Don't get discouraged and be sure to stay involved here and the people will help or try to help you resolve the issues that arise. Most of the weaknesses and strengths have been identified for all ranges of performance and riding styles by the members here. I am lucky to have had friends and this site to help me get my KX5 running like the bike is was meant to be.... the ultimate dune bike....
Sly