1) All instances are available for 6 man parties...only specific ones are available for 12 man raids, and a select few can be done with an entire raid(36 people). Hard modes and HoS are 12 man instances, and VN and BG are open to anybody who enters(you don't even need to be in a party)...
2) the duel class system works like this: you pick a main class,, at level 10 you pick a 2nd(and lvl 20 u can take a 3rd). you have a primary and secondary class, you get all skills from your primary, and all the "general" skills from your secondary, plus that specific combination of primary/secondary's elite skills. each class has to be leveled individually and each class must be normally obtainable for your race.
3) cash shop
items are pretty much required, but it is not necessary to buy them with real money. paying is not required, it just makes everything a heck of a lot easier...and if f2p is your thing, you can always buy CS items from other players for ingame gold.
4)level cap atm is 80
5) any other information can be divulged...if u have a question ask it, which i think you already know