[Culture20]

In point-buy systems, there are always ways to "cheese", and GURPS excels at this. A common example is an innate attack that does only 1 point of damage with area effect maxed to the gills, and possibly other limitations. You end up with a character that can kill universes slowly, but within budget.
It's one of the big reasons why character creation needs to involve the GM. You specifically mentioned wanting to play a "more realistic" game but with magic, so your GM might have ideas about what the limit of magic should be.

Also, as the good doctor alluded to: reductions in attribute levels from default are usually considered as part of the disadvantage limit (Although I allow my players to purchase "partial" IQ without Will and Per; Holmeses are quite rare even among fictional geniuses).

Finally, did you consider your character as a somewhat well-rounded person, or is he solely devoted to magic with no real skills? Sure, you can default IQ skills pretty well or at least the ones that can be defaulted (stealth can default to IQ!), but It would make sense for your character to have spent some time doing something other than practice magic.
Edit: Something that isn't covered well in the books is you need different innate attack skills for different spell types (throw, jet, gaze, breath). Fireballs and magic missiles in D&D "just work", but in GURPS' base magic system, you have to roll DX based skills to throw or aim your attack spells (or spells that you meta-magic into a thrown version).