Hi,
I 've owned a TXT250 pro model 2002. It's a very friendly, light and nimble bike. It's absolutely a great bike for starters.
I never had any major problems. What did break was the return spring from the kick-starter. I think Gasgas resolved that problem by 2003, so that should not be a problem.
What could be a problem is the clutch. The hydraulic system always tends to leak in the engine case. That causes bad clutch action due to air in the system. I think that this problem was fixed on the 2005 model by changing to mineral oil in de hydraulic system of the clutch. On my 2002 bike I swapped the complete hydraulic clutch system (that included the right side engine case) for 2005 parts. It solved the problem.
Also the stock Sachs rear shocker wears out quickly. I tossed it for a WP unit within two years, because there was no compression damping left (I'm not kidding). The WP was miles better then the stock shocker also. I now run an