Gydion: Just that I prefer to play pure class builds. A magic using Paladin can be a very effective character.
JMich: A Paladin is a magic using character unfortunately, certainly in QfG4, I think in QfG3 as well. First time I had to fight a wraith with an untrained aura spell instead of an aura medallion did make sure I'd remember it.
If I recall correctly, that is changed in QfG5, where it used stamina instead of mana, but memory is hazy.
In QFG3, upon learning your first Paladin spells you instantly gained 5 points of magic. Although Paladin spells use stamina instead of magic, you now have a magic skill rank and a certain number of MP. This was presumably a hack by the developers since the magic option in the GUI was greyed out if you had "0" in the magic score. This behavior is also the same in QFG4. The end result is that any Paladin in these games invariably gains the ability to access wizard spells.
In QFG5, this was fixed and a Paladin was no longer required to have magic to cast his stamina-based spells. The Paladin import file provided with the game conforms to this notion and has "0" in his magic skill. When importing a character into Dragon Fire, cross-class skills with very low skill levels are either boosted to 100 or dropped to 0 (I'm not sure what the threshold is) so depending on how you played an imported Paladin may actually lose the magic ability that was added in the earlier games. Seeing as some amount of training of the magic skill is required to use the aura spell effectively, it's likely that any Paladin that legitimately played through QFG4 would indeed have a non-zero magic skill when imported to QFG5.
People frown upon cross-class skills in the QFG series because there actually isn't much difference between the classes if you do it, which limits replayability. Depending on how many times you wish to play through the series, that may or may not be an issue. If you only plan on one playthrough, then use cross-class skills all the way. Just beware of being a Paladin with the pick locks ability - it'll royally screw you over in QFG5 (though a save-game editor can save you here; all you need are a few pieces of thief-specific equipment)