much is it to buy? - As they said above it's about £30 depending on where you get it from
can you pvp on it / kill people? - Of cause you can, thats the main part of the game, as stated above there are 2 factions (Horde + Alliance) and there are various good pvp features (for example people of the same faction can "duel" against each other - you don't die, a pvp ranking system - honor - which you can get either by killing people in the normal lands, or by participating in Battlegrounds - another good featue where a horde team and an alliance team go against each other to do certain objectives - for example CTF)
easy/hard to lvl? - It's pretty easy to level up - the maximum level is 60 and there are various quests etc that you can do, and a lot of places to level
what class best for good damage and fighting? - Hmm well my knowledge isn't perfect but there are various "good damage classes" - if you want close range attacks i'd be a warrior, if you want long range attacks i'd be a Mage, and if you want both i'd be a Shaman - all of the other classes have various benefits, and you've got to remember certain races are better at certain things.
what pc spec would i need to playt this game okly..? - Erm well you need an okay pc, nothing spectacular, the only thing that really caused my game to lagg was when you're in the major cities (SW and IF especially) - sometimes it even caused it to crash. I suggest you check the site for a minimum spec guide etc.
I'm not exactly the foremost authority on WOW, i used to play it a while ago, and i may pick it up again =)
/Miles