[Crenshinibon]

I actually used an Assassin/Cleric build before. That's really something. The to hit and damage bonus is nice and you can do some impressive damage with that backstab of yours.

As for spells:
1: Armor of Faith. 25% resist physical at level 20. It stacks. And even if you don't want to be cheap, you can get yourself the Defender of Easthaven
2: Ehhh... Holu Might if you're desperate. Good at the beginning though.
3: Animate Dead (the after level fifteen version), Glyph of Warding (The cleric skull trap) and Holy Smite. All good.
4: Holy Power. It stacks with itself. Got me a seven hundred point priest for a few rounds. Doesn't stack with Tenser's though. Tenser's doubles your original hitpoints.
5: Champion's Strength (if you're lacking in THAC0, beware of the strength reduction, it overrides everything else. Righteous Magic. Nice strength bonus but we want this because for the duration you deal maximum damage. This is like what Kai wants to be.
6: Eh. Play with this. Improved Haste + Blade Barrier + Sanctuary. Try it. You're not the one attacking them so Sanctuary won't break. Also, Improved Haste affects Blade Barrier as well. Maybe use the summons early on. Oh yeah, and Harm. Don't forget harm. If you have the THAC0, and the creature is living with an anatomy, well... they won't be too happy afterwards.
7: The Deva is a wonderful summon for priests. If you're more combat oriented, get Aura of Flaming Death for the AC bonus. Regeneration stacks with itself and is affected by Improved Haste.

As you can see each level has something that's worthwhile.

So for a Cleric to Thief (I recommend a Priest of Lathander for the ability... which also stacks with itself and Improved Haste)

I'd go for the maximum and do 26 priest (4050000) to 27 thief. If you want to take my advice and do thief first then I'd say dual class at 24 (3080000), after you get UAI, which would bring you to a 29 cleric in the end.

Hope this helped.