The idea is, that because two handed weapons are so slow, you can increase dps by increasing dexterity. However, this could possibly limit you to lower level weapons if you put in too much dex, and personally I think that it's better to have a single handed weapon, a shield, and a decent dex. This will keep your dps relatively close to what it was with a 2h weapon (within 1 or 2), as well as giving you whatever bonuses the shield provides.

It seems to me that because the purchases are linked to both your PL account and your iTunes account, the server will "remember" it and you will still have all your expansions.

You do not have to buy it for each character. Once you have unlocked a campaign, any character you have, regardless of level, can play there.

25 is the current level cap (if you have all the expansions), but there will be more expansions coming out soon that will raise the level cap, so relatively soon the cap will be 35.

I hope this answered your questions