you shoudl get platinum edition: eye of the north and prophecies, then if you like the game, get nitfall, then factions. in that specific order.
chapter 1 prophecies: has all the core classes that are the most requiered, has a good training for a new player. it is directly linked to chaptrer 4 (eye of teh north expansion) which came out last and this is where the majority of the player base plays now. you cannot use more then 3 heros at a time, and you get a good bunch of them in ch4, so IF you like the game then you can buy NF (ch3) and then ch2 (factions)
chapter 3: still has players playing in it, but the "assemblies" for all the fun stuff like dungeons and elite areas alays happen 90% of time from ch 1 and ch 4. also ch4 has some very important things that chapter 3 and 2 dont have, and you will want to have access to them:
1. consumables
2. dungeons
3. a few (or shall i say 1? ursan lol) skills that you will need to get into the party of random players
4. weapons that sell for alot and that might make you ritch
most player base hangs out in 3 towns in chapter 1 and in chapter 4, most people dont come back to do ch 2 or 3 unless they have to to get a specific skill, armor or a title. the for fun play mostly happens in ch4 or a a few areas in ch 1 (temple of the ages as exmple is the main reunion place to team up with people doing fissure or underworld, which are teh two most profitable areas. you can enter those from ch3 and 2 as well, but there wil be MUCH less party options since most people look for party from TOA)