The math appendix of the
Beyond Divinity Strategy Guide gives a description of the effect of the main stats, and some general suggestions for stat point distribution.
The beginning of the game is probably easier playing warrior type characters, though once you get up a few levels you can start developing one or both main characters as a mage or survivor. The more powerful spells, particularly shaman magic, start to become available in act 2, so you might want to equip a mage with a bow as a backup or supplement to magic.
Your chosen character class merely determines your starting skill point distribution (or you can use a custom distribution) and the initially available skills. You may develop your characters however you wish after that.
There are no restricted character types, so it would be difficult to say one particular build has benefits over another. I played the beginning of the game with both characters melee warriors, then switched one character to a full time archer (the
patch makes arrows cheaper and more plentiful, among other things). Having one tank warrior and one ranged character (archer or mage) works well, but people have also played 2 melee warriors successfully. Two ranged characters might run into problems when they have to fight close range.
The DK's armour class goes up with his level, so you may want to make your hero a warrior, and the DK an archer or mage (that way you can neglect the DK's strength in favour of agility for an archer or intelligence for a mage).
There are some strategies for the start of the game in the topic
Killing Fergus, though I would wait until you are up to that point to read it, to avoid spoilers.
As for the demo, check if there are any updated drivers for your video card. This was more of problem with the full game than the demo, but updated drivers have fixed similar lockups.