I'm only a level 10, so take what I say with a grain of salt ;)
For both mages, get Withcraft to level 2 fast. Rapture, Oath of Desecration, and Drain Willpower are very useful right away. Rapture actually feels like cheating. There are some other low level witchcraft spells that are solid.
For your marksman character, get a point or two in Scoundrel for Walk in Shadows, Winged Feet, and Fast Track. There are some other skills that are sometimes useful like Trip, Adrenaline, Become Air, Cloak and Dagger, etc... With Walk in Shadows you can steal paintings and weapons and open chests in front of others and not get caught. You can also enter sneak mode and not be revealed in front of others so you can pickpocket a vendor (using +1 pickpocket gear).
For your Loremaster character, you only need to put 2 levels in it. You can get to level 5 with two +1 rings and a +1 amulet.
If you want to put Blacksmith and Crafting on one of your mages, it's not too hard. The Scientist Talent gives +1 to each, a belt can give +1, and a bracer can give +1. You only need to waste 6 ability points to get to level 5 in both. I believe there are two books in the game that can give +3 ability points, so if you use them on this character he won't be too nerfed.
A few other tips... Don't waste points in Charisma. It's not that great. Check vendors after each level up, they restock. Buy every Sinew or Strong Sinew you find. These are needed to make Bowstring for making and enhancing Bows. Buy every Pixie dust, Stardust plant, Sheet of Paper, Charm and Silver arrows/arrowheads, Tormented Soul, Ore, Iron/Steel/Silver Bar, Ruby, Moonstone, and if you can afford it the Water/Fire/Earth/Air Essences.
