*Edit* Ah, see the spell slinger bit and opposition school stuff now... man I need to stop posting to forums while working..
Some actual, real helpful response...
Opposition Schools: Remember that while you might be focused on ray spells, you gain benefit from cone, line and ranged touch spells as well with Spellslinger.
Likely choices for opposition: Enchantment, Illusion, Divination and Abjuration.
While some of their spells are nice, and occasionally fit the ray/ranged touch, they are few and far between. Ultimately, it's a major hit to your magical toolbox, but it's not an insurmountable one (it's only 2 spell slots and a hit to spellcraft checks, get scrolls and wands for troubleshooting spells).
Maybe pick up Abjuration for Opposition candidates as in later levels you may need better access to defensive magic for survival.
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Gunslinger Levels: Since you get exotic firearms proficiency with Spellslinger, you can take a Gunslinger archetype without losing proficiency (and thus versatility).
Since you want single pistol, taking Pistolero might be a good idea.
You may even ask how your DM might rule the Up Close and Deadly damage with regards to spells cast through your gun.
Was there a reason for the Eldritch Knight levels? Touch attacks don't need high BAB, and you get enhancement bonus to your spell's attack rolls and saves, so you don't need the high BAB so much. Was it access to Weapon Specialization?
Note that every 4 Gunslinger levels gives a combat feat too. I'd take at least 5 levels of Pistolero for the Dex damage, up close damage, and feat. Better skill and saves too.
Pick up Rapid Shot (since you can do it with Rapid Reload + Alchemical Cartridges).
Arcane Strike is nice too, as long as you don't need your swift action for anything else. Better than toughness.
Signature Deed on Up Close and Deadly and you only ever need 1 grit on hand to get your +2d6 damage (and damage even with a miss).
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Stuff to Get: Well for one, at least one of every bullet, and specialty stuff if possible (silver, adamantine, etc). With Abundant Ammunition, you won't need to waste money on ammo (as long as you prepare a round), but you need them on hand.
With Spellslinger, you can use scatter weapons fine; entangling shots and flare shots can give you back some versatility. Have See Invisibility up (via wand or scroll) and tag an invisible opponent with your flare shot. Now everyone can attack it fine.
The Slate Spider (10k) can get rid of miss chance completely for you (free action, so allows full attacking it seems).
Lucky enhancement might be a better solution than Extra Grit if you have more gold than feats.
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Hope some of this helps in your decision making!