Don't be a sorcerer, the fact that they cannot learn scrolls makes them next to useless as spellcasters in my opinion. A dualled cleric to mage would be fine, but I would reccomend waiting until level 11 before dualling, and the downside of this is your mage will not be able to reach high level spells. The necromancer is probably the best route to go, for sheer spellcasting power at least.
PS. I am not experienced, but I thought I'd answer this anyway.
