With Dark Ascension's release, what changes would you make to this (budget) deck I'm considering purchasing

If you're making a budget u/w tempo delver deck then you should focus your strategy on that (drop early threats, back them up in following turns with disruption a.k.a. counterspells/removal/bounce). Following this strategy, you should cut the soul seizers (it is not an early threat and hardly disruptive). Also, if you can afford runechanter's pikes (you can probably get them for $0.50 or $1 each) use those instead of the current equipment you have. Or use silver-inlaid dagger main-board. If you decide to use the pike, cut the worst creatures (something like -2 elder cathar, -2 gideon's lawkeepers, and -2 soul seizers) and play some 1 mana cantrips (ponder, thought scour, gitaxian probe). Ponder could be helpful especially for triggering your delvers and thought scour can get rid of cards off ponder you don't want.

Realistically, the delver deck works best with more expensive cards like snapcaster mage, and dual lands (makes sure you can play your spells consistently without waiting to draw the correct land, effectively losing turns). If you wanted to play more creatures than spells, I'd suggest going human tribal fully and losing the delvers, soul seizers, etc, and put more humans in in their place. Mainboarding the silver-inlaid daggers would also be good in this sort of build.

tl;dr don't play soul seizer in this deck. Delver of secrets needs a high instant/sorcery count to work well (best with cheap cantrips). Choose one strategy and stick to it (don't mix themes in your deck).

The glacial fortress is in there just because I already own one

[deleted]