Mages are always offensive to those of us without Int ;)
First, stats. There's really two options for you to choose from, same as with your warrior: Int, Dex, Cha (bonus accuracy for your pet+guest) or Int, Cha, Luck (initiative + LS for big numbers). Most people go for the latter, but if you're running with dual Mosquitoes, beast bth might be more useful. In the end, the choice is entirely up to you.
Next, subrace. Vampire, no second thoughts about it. Qc stun, Int boost, Evolved Form with minimal upkeep that heals hp based on damage dealt. You won't be using it nearly as often as Necromancer, but it's still very much worth maxing out.
Class Title: Necromancer. For a beastmage - or any mage, really - there's just no competition.
I'd probably go with an Earth no-drop, as it has no shields worth writing home about. Don't forget to switch your Blade to Staff of Awe!
FO mage has the capability for highest DPS among all builds, but it also requires the highest investment to fully min-max. The best weapon - for every element - is an Arcane Cutlass, which boosts your spell damage by 30%. Unfortunately, they also cost a UR each. Just keep in mind that each weapon I recommend below comes with an unspoken addendum: 'if you can't afford an Arcane Cutlass'.
Blazing Bloodzerker's Sword boosts both weapon attacks and fire spells. To be replaced with Pyromancer's Bloodblade when Caecus returns to the Void for Bloodmage FSB.
Multimaul for Berserk (great for boosting spells of all elements). You can also use Blade of the Briar if you want to play around with Vampire Tranformation; Bleed restores your hp via Drink passive, and in Evolved Form the skill deals high damage.
Both Feather Buster and Kazemai Athame provide small buffs to wind spells and spell-type skills. Or you can just keep MADM for dodgeplay.
Your optimal ice weapon depends on what armor you'll use. If going with Cryo Bloodmage or Necro Cavalry, Arctic Athame gives +18.75% bonus to ice spells, with bonus doubled against foes with Int. If going with Horo Show, Mystic Blade of Frostval Past would be more useful.
Thunder Lord's Staff gives you an autohit option.
If you get Lumenomancer Bloodmage, Mystic Retro Golden Axe would be very helpful. If going with WKZ... well, its skill doesn't apply weapon effects, so you might as well get something like Loremaster's Tome for harm and light (when using necro) spells.
For darkness, you want to maximize the damage of your Necromancer spells, so Blackbones is pretty much your only choice outside of a cutlass.
Pyromancer Bloodmage is the only non-GGB Bloodmage in the game, and 100% worth getting the moment Caecus gets to the Void.
Fall Dryad's Form gives you spellcaster armor with earth/water compression and a compressed spell. Unfortunately, it only returns on Grenwog, so until then use just Quenching Bloodzerker and Eternal Champion's Spirit for water and earth.
You'll have to use Vampire Transformation as your wind armor until Fall Dryad frees up a slot for you. Afterwards, Anemomancer Bloodmage would give you the highest DPS; Mark of Wrath if you can't afford a UR.
Cryomancer Bloodmage gives you the highest DPS; some instead favor Ice Necromancer Cavalry despite its lower damage because it lacks an hp cost. Both of those are URs, though, and Horo Show will have to tide you over until you can afford to splurge on one of those armors.
Energy is the only element that will suffer DPS reduction when you switch over from warrior to mage. Dynamancer Bloodmage comes the closest, but it's also a UR. The next best option is Taladosian Robes.
Paladin drops off sharply once you lose the class title, so the best replacement would be Lumenomancer Bloodmage. Again, a UR, with WKZ being the next best choice.
Necromancer is the best mage armor. It has spellcaster lean, Int drive, a variety of nukes, several resource regen options, passives and toggles to enhance your spells - and that's not even getting into 6 guests, 3 pets, resurrection, and all the other bells and whistles it brings to the battlefield.
Replace Chieftain's Ironthorn with Celtic Wheel for Int drive.
Serenia's Insight instead of Mana Crystal Ward. Its Int drive is more useful for a fully offensive build.
TropoShield in place of Haunted Dragonlord's Will. Grab it before Black Friday discount ends!!!
Replace Dunamis with Poelala.
Essence of Carnage loses half its effect if you're not using melee/ranged weapons. Switch it with a dedicated Backlash guest like a Fiend of Nulgath, or use Gandolphin for a little extra survivability.
Replace Dunamis with Poelala.
Use Z-finity Gauntlet of Power or Grakma Warhorn (as Berserk doesn't divide /2 for spells unlike most other damage boosts) instead of Blood Contract.
Switch Urn for Pizel Ether or Rejuvenating Necklace for mp recovery.
...and that's about it, I think.
Hope the info's useful, and don't hesitate to ask if you have any questions!