Necro twink on 25k budget

[soup]

Black Sapphire Jewelery, HBC, Plat Fire rings or Platinum Skull Rings, Rokyls Channeling Crystal, Platinum Tiara, Golden Jaded Bracelets, Dagger of Marnek, Cone of the Mystics, Shadowbound Boots, Embroidered Black Sleeves/Mantle

You should be able to pick up all that as well as minor stuff for the missing slots for your 25k budget easy. All that gear is heavily focused on HP and raw mana (which will heavily outweigh INT for a long time) and past that, not much you can do to make life easier, unless you have enough of a budget to get a Rod of Annihilation.