A couple of thoughts:
For the massive undead army, you can use Wall of Iron + Fabricate + Craft boosters to make tons of basic metal armor and weapons for them.
You can get away without paying for some items through long-lasting spells - for example, Superior Resistance negates the need for 50k spent on two Cloaks of Resistance +5.
Obviously, a cohort focused on item creation (artificer...) effectively halves the cost of items for you, though it's a bit of a cheesy way out. You could also get a cohort who is largely item-independent or self-buffing, like a bard, cleric or druid (though undead doesn't play well with druids...).
It's not like wizards are that expensive to equip, anyhow. Assuming you don't buy any tomes or staves, you'll probably have everything you need by 400k, with 40k of black opal and another 320k to spend on your cohort and army.