Greetings

1. Euros have a bigger potion delay, but they have (generally speaking) more powerful skills than Chinese (think about DareDevil, Wizards with life control on, rogues with Dagger Desperate / Crossbow Extreme). Chinese are meant to level solo, whereas euros are designed for party play. An euro levels much better by partying and some builds have a hard time solo-ing.
2. Yes, you should be pure INT. Some people play light INT hybrid warlock/clerics, but that's not really verified yet. It might be an alternative, but pure INT is basically the way to go. Having Cleric or Bard as your main class makes you a support build. Most people play Cleric/Bard (without having one of them as a clear subclass: they max out both).
The most popular build choices are:
- Wizard / Cleric (better for party play, as opposed to Wizard / Bard, which does better solo-ing)
- Warlock / Cleric
- Warlock / Bard (another superb solo build)
3. PvP is very gear dependant. However, INT euros tend to have the following common trait: they either kill fast or get killed fast. For example, if a blader blocks a wizard's nuke and the wizard is in life control, the latter will be 1-hit by a blader skill. On the other hand, the wizard can 1-shot the blader with ease, should he not block the skill.
Stealth and invisibility is another factor that renders euros exceptionally powerful, not to mention Bless spell and warrior skills (skins, Daredevil, shield crush etc.)
4. Gaia should be a good server to get you started. You can also try Venus, there's a big legit community over there.