Some of the questions you asked about levelling have already been answered, some have not.
To wit: enemies and gear scale with you, so if you restart at level 45, you'll have much better loot lying around than if you started fresh from level 1.
If you restart and pick a different class, all the skills and spells you had from your other class are locked. There is, as far as I understand, currently a bug if you try to pick the same class in a new game.
As far as "how high can I go before I cap" sort of questions, my advice is play and find out.
The higher you go, though, the meaner mobs get. When you get your party again, they will be in gear that's not all that amazing, and this can cause them to get kicked around pretty hard if you're not careful. Your party will be at whatever level they ended your previous game at, but they'll have all their expertise and attribute points back, so you can re-spec them. Remember, though, that if you, say, pick an elemental attunement with Gwen, she'll still only gain one spell per level, even if she's high enough to max everything in a given skill tree. Goes for anyone else too, for that matter.
Check out my Manamon text walkthrough at the following link:
https://www.dropbox.com/s/z8ls3rc3f4mkb … n.txt?dl=1