Thinking of buying this car used between the years of 2000 to 2002. Does it use a timing belt or chain? Is a chain better than a belt? How often would you need to change either, and what estimate for the cost?
Any other remarks about this g20?

G20 has a Timing chain.

Timing chain is a better choice for long service life, it can last a really long time. But it is more noisy, heavier and less efficient than the alternative of timing belt.

The alternative of timing belt, which G20 does NOT have, but some other comparable cars do, is that it is cheaper to make, and maybe lighter and more efficient for power... but it doesn't last as long, and if you don't replace it at the right time (perhaps 80k-100k miles for Dodge 2.2L, the only one I know from experience) then breaking of the belt means fatal problems when pistons hit valves left stuck open.

In comparison when a timing chain gets old, maybe it gets a bit loose and sloppy, and valves open a little slower and later than they should, but they don't hit pistons and don't blow your motor it keeps running but not as efficient as it could be with some service.