As said above, Gas Gas enduros are basically a strong, sound bike, even the older ones. However, our climate is hard on dirt bikes and older bikes generally accumulate more problems. It's the small items that can quickly add up into large bills and make buying older machines uneconomical.
I got caught out last year buying my '06 ec200. The bike was reasonably tidy looking, and I thought it was basically ok, just badly jetted. Driving home with my new purchase in the van I realised I hadn't checked for up / down play on the crank, and it turned out failed main bearings accounted for what I thought was poor jetting. Add piston kit, mains and seals, gaskets, several gearbox bearings all other the various wheel and linkage bearings, fork seals and bushes et c. et c. before it was in decent condition, and the hardly used '09 bike I had been offered suddenly looked like much better value for money.
But there again, you might get lucky and find a tidy older bike for a low price....