At least for the beginning of the game, your mage will probably need to have some melee or archery skills. You are correct that the DK doesn't need as much strength, but the spells you start off with are a little mana intensive. More powerful and efficient spells (especially shaman magic) start to become available in act 2.
I started off with 2 warriors, with one using a bow occasionally (for tough fights). Towards the end of the first act, I switched one character to a full time archer (arrows became more plentiful, and there were much fewer skeletons, who are better dealt with using crushing damage).
There is food in the north west cell in the starting cell block. You can find a bedroll shortly after getting through the first level, which will allow you to sleep anywhere (there are several haystacks you can use before then).
I never had to wait around long for mana to regenerate, even with a relatively low intelligence (though I didn't use many spells). I slept less frequently as the game progressed, and stopped entirely in act 2. With a mage you may need to sleep more often (until potion availability improves), but as you level and boost your intelligence and survival, the mana regeneration rate will go up.
I did have to stop running frequently in the beginning to let my stamina regenerate, but that became much less frequent as the game progressed and my characters' constitution stats improved.
Your choice of character class at the start of the game only determines the initial available skills and spells. The choice of either custom or predefined stat distributions (for warrior, survivor and mage character types) will effect the very start of the game, but after a few levels it will not make much difference, since you will be able to adjust the stats however you wish.
That said, it may be easier to start over, since you are so close to the beginning.
Killing Fergus - strategy and beginning stat distribution
Kiya's BD demo
screenshot walkthrough/spoiler covers the first level of the full game.
Beyond Divinity Strategy Guide - the math appendix contains a description of how secondary stats are derived from the primary stats, and gives some general advice on stat point distribution