How much of my WBL should I spend on spells?

[Jeff the Green]

I'm statting up a Beguiler/Archivist/Mystic Theurge (yes, I know it's not terribly optimized; yes, I'm using early entry), but I haven't played a wizard or archivist before. We're starting at level 7, and I have 19K GP to spend (16K after necropolitan). How much of that should I spend on spells?

[Pilo]

You need at least:
- a headband of intellect +2 (4000gp)
- a +1 chain mail (1400 gp)
- some Pearl of power, 1st-level spell (1000gp each), 3 may be enough.
- a wand of vigor minor (750gp)
- Cloak of resistance +2 (4000gp)

It costs you aroud 125 gp to add a first-level spell to your archivist spell book (Some scrolls are more expensive than others).
You already get 5+Int Modifier first level spell (lets say +3) so 8. 4 or 5 more may be enough so aroud 625 gp.

It costs you aroud 350 gp to add a second-level spell to your archivist spell book (Some scrolls are more expensive than others).
You already get 4 of them. 6-8 more may be enough so aroud 2100-2800 gp.

[Malachei]

You have a great selection of Beguiler spells you can cast spontaneously.

I'd focus on acquiring low-level scrolls, scribe most of them and perhaps get a Runestaff or two, and perhaps an Eternal Wand of a 1st level spell.

I wouldn't buy a Headband of Intellect, because I'd guess I'd find that in-game eventually, at least if you can expect to be fighting enemy spellcasters.

I'd rely on mage armor (or buy a chain shirt) instead of +1 chainmail, and I'd prefer having a shield spell ready.

Unless your DM rolls treasure randomly, you're likely to find +1 items, such as resistance, armor, weapons early in the game.

[JeminiZero]

I'm statting up a Beguiler/Archivist/Mystic Theurge (yes, I know it's not terribly optimized; yes, I'm using early entry)

Don't be fooled, that build has monstrously powerful spellcasting potential based on one feat: Versatile Spellcaster. This lets you cast any spell you know by burning 2 slots of a lower level. Keep in mind that:
a) The Archivist knows all the spells in his prayerbook, and
b) Nothing prevents you from burning Beguiler slots to cast Archivist spells.

[Jeff the Green]

Yeah, not going there. I want a nice, comfortable tier 2-3 build. That's good to know if I'm ever doing a higher power Mystic Theurge.

[Alefiend]

Yeah, not going there. I want a nice, comfortable tier 2-3 build. That's good to know if I'm ever doing a higher power Mystic Theurge.

Having the option is not the same as being overpowered. You can play at your comfort level, secure in the knowledge that you can pull out some neat tricks if the adventure ever goes pear-shaped.