Here's what I threw together. I don't have details on crafting said golem because I'm not well-versed in item crafting costs and so forth.
Hit Dice: 24d10+30 (150hp)
Armor Class: 24 (-1 size, +1 Dex, +14 natural), touch 10, flat-footed 23
Attack: Slam +23 melee (1d8+5)
Full Attack: 2 slams +23 melee (1d8+5)
Special Attacks: --
Special Qualities: Construct traits, damage reduction 10/adamantine, darkvision 60ft, immunity to magic, low-light vision, object polymorphing, spellsource
Saves: Fort +8, Ref +9, Will +8
Abilities: Str 20 Dex 12 Con -- Int -- Wis 10 Cha 1
Challenge Rating: 17
Alignment: Always neutral
Advancement: 25-47HD (Large), 48-72HD (Huge)
Level Adjustment: --
A hulk of briliant blue stands before you, illuminated by the steady glow of its eldritch power. Its form is generally humanoid, though its eyes lack the luster of intelligent beings. Suddenly, with single-minded fervor, it rushes toward you.
This golem is crafted out of special rare materials, and though it serves as a capable combatant in its own right, it is more useful to its creators for its spellsource ability. Its skin is of mottled topography, but evenly lit with a soft blue light. It moves determinedly, pausing briefly between each action as if to punctuate its efforts.
Basal golems typically weigh around 400 pounds and reach heights of 10-12 feet.
Basal golems fight with their fists in their base form, but are more commonly used by their creators as backup sources of arcane power or as an intermediate step in obtaining some other creature or object.
Immunity to Magic (Ex): A basal golem is immune to any spell or spell-like ability that allows spell resistance. In addition, certain spells and effects function differently against the creature: Any spell of the polymorph subschool cast on a basal golem paralyzes it for 1d6 rounds. Exposure to an antimagic field slows it (as the spell) for as long as it is exposed and deals 1d6 damage each round. If it is targeted by an imbue with spell-like ability spell or effect, it is hasted (as the spell) for 1d6 rounds. Finally, any dispelling effect (including disjunction), in addition to functioning normally, deals 1d4 damage to the golem per the level of the spell in question.
Object Polymorphing (Su): If commanded to by its creator, a basal construct can begin a process of metamorphosis. (It never takes this action autonomously.) It coats itself in a thick protective resin which instantly hardens into a dark red-gray chrysalis; while so entombed it cannot move or take any actions, but gains DR 20/adamantine and magic and its natural armor increases from 14 to 24 (new AC 34/10/33). When it emerges, it is affected as if by a polymorph any object spell, changing into a creature or object of the commander's designation (decided when this transition first begins), except that the HD of any target creature must not exceed the basal golem's own HD, and that multiple basal golems who are adjacent to one another can use this ability in synchronization with one another to greater effect. For each additional basal golem in the chrysalis, the duration factor increases by one. Thus it is possible to combine many golems together to create great structures such as castles or ballista permanently. The metamorphosis initiates as a full-round action, and takes a number of minutes to complete equal to 10 minus the duration factor total (see the spell description).
Spellsource (Su): The basal golems body was crafted from arcane energies, but was not fully infused with them, and thus can be used to store additional spell energy which can be later retrieved. The golem's body acts as a ring of spell storing, except that it can store a number of spell levels equal to its hit dice, and that the stored spell levels can only be retrieved by a spellcaster who created or currently owns the golem. Retrieving a spell from the golem stuns it for 1 round.
Edit: the whole 'not moving while being tapped for mana' thing.