1) Yes the classic version is probably the best if you're trying the game for the first time.
2) You can level without siel, but you will have a lot of limitation including for loots. Given you will probably level solo, believe me you want better gear than grey/white loots or you will get smashed by mobs.
Siel is pretty much mandatory and as for more $ needed, if you want to be competitive yes you will have to spend a lot of money even in classic (or farm instances 8 hours a day - because inflation from RMT makes everything very expensive if you grind instead of purchasing kinah).
If you don't care at all like I do, you should be fine and progress at your own pace, just expect to die in pvp..a lot 
If anything try it for the combat system during leveling, it's really the best of this game i'd say.
3) Sure, but make sure to choose Atreia as a server, Taha is quite dead.
4) Never played on NA I can't answer on that, but NA is more advanced in patches and has less players.