First of all, Tomb of Horrors: Is this the WotC third edition Tomb, or some sort of revamped version of the Tomb made by your GM? In the case of the former, you really don't need to optimize, the dungeon is actually rather easy going. I ran it for a party of two and they ultra-stomped it with half-way optimized characters, except for one trap which cost them a single party member due to poor decisions and poor rolls.
I second DMM: Persist, though I will note that my interpretation would not allow you to persist an extended divine power to result in 48 hours, as DMM: Persist sets the duration of the memorized spell as you cast it to 24 hours. DMM: Extending a memorized persistent spell, or DMM: Extending a DMM: Persisted spell would increase it to 48 hours, however that takes another feat and more turn attempts. I'm not saying that I'm right, just that you should always ask your GM before writing your character sheet in ink. But really, go crack open a Complete Divine and take a look at Divine Metamagic, its probably the route you are expected to go anyway. I'd also recommend taking the ACF "Spontaneous Domain," as it is almost always better than converting spells to cure spells, and then talking to your GM whether casting substitute domain (Complete Champion) would allow you to spontaneously cast spells from the substitute. The RAW is hazy, but I have always ruled for it, and if so, select undeath as your spont. domain and substitute it for any domain you please after persisting your spells for the day.
For advice a little more ToH specific, I would strongly recommend two levels of Divine Oracle, persisting Delay Death from Spell Compendium, and preparing at least one Swift Etherealness (PHB2). Let your party members do everything for you outside of combat; if they ask why you don't open doors or take point explain that you can bring them back from the dead, they can merely drag your corpse back to town, if they even make it that far without your help. I know it sounds like a dickish thing to do, but I've had more parties wipe to random encounters on their way back to town after they insisted their party cleric was "A CoDzilla" then I'd care to recall.
Addendum: I missed the part that you claim to be bad at making characters. Point buy for a cleric can be rather difficult, actually. The first problem is whether you intend to cast offensive spells or not, and whether this campaign is a one-off adventure, where you'll make new characters later, or an ongoing campaign. If you intend to cast buff spells, heal spells, and res spells more than casting directed spells that allow a saving throw, and this character has no potential to reach 17th level or so, Wisdom actually becomes less important in comparison to other stats. In this scenario, I would consider something like 8/10/12/8/16/18 with stat bumps to charisma. Actually, further input on what min/max thinks a good 32 point buy for a cleric is would be helpful.
