1. This server is not pay2win in any means. You can play without spending real money and take pleasure from it. You just need to remember that this is Classic and it's a lot harder than on "modern L2" servers.
2. The best class is that, which you will like to play. Every character is better or worse in some aspects, one are a lot harder than others, but still all is definied by your playstyle. if you will get a good clan, there should be not problem with parties. No matter what class, you will find a RB if only you have a bow. For sure I can propose dwarf character as second char for making adena.
5. Playing without dualbox is possible. For small money you can buy buffs from high lvl PP in giran harbor (free teleport). It takes few minutes, but you will get a lot better buffs that from some dualbox. I'm playing a spoiler, close to 40 lvl. No dualbox and there is no problem with exp. And spoiler isnt a better EXPer (?) than other melee clases. So you can play without dualbox, its need just a little more adena spending.
That are just observations of low lvl player, who started not so long time ago.