Anything 2001 or newer should be aluminuim floored which is better than wood. No idea about values I'm afraid, they hold their value so well there often isn't a lot of difference between them, new or old!
Make sure you take someone knowledgable (mechanically!) with you so they can poke around at the floor, brakes, electrics etc.
Our is a 97, it's been well looked after, lightly used for the first 10 years, then regularly used and maintained by us. The floor has been fully replaced and there are currently a few wee jobs to do, leak in roof, replace a corroded wire. But apart from that, it's a sound reliable trailer.
We were recently offered £1500 for it, but decided against upgrading.