Yes, if you have a mage class, rest at the first opportunity, but make sure you have spells in all the available slots in you mage spell book. Only the ones 'memorised' will be available for use, so be careful in your choice.
Any class is good; but I generally make a point of creating a
human fighter and then dual classing to the class I want (make sure your human character CAN dual to the class you want first! (Check out the available multi-classes in the manual).
In creating your fighter, bear in mind the attributes your dual class will need, such as 18 wisdom for a Cleric, or intellect for a mage, and only give weapon skills your dual character can use. This way you can get fighter level skills with hammer or mace or staff or sling, etc, that is not available to the straight Mage or Cleric, or whatever - besides fighter health points!
I dual as soon as the opening set scenes are finished and Imoen has done talking. That way, you should equal your fighter level by the time you exit the first dungeon, surpass it soon after, and have your fighter skills available again.
Good luck!
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