Hmmm, I will try not to spoil anything for you, but assuming you are going to be up against Drow, be careful with a Buff-dependent build, because Dispels _will_ occur.
Try not to waste a high caster level - it will be needed both for high level spells and spell penetration (as Drow etc. have SR ) - and of course turning efficency. A single fighter or ranger level should set you up with needed proficiencies. On the other hand, consider playing an Aasimar, who's bonus to Wis and Cha will certainly be useful with a cleric.
Given that the Drow are Underdark dwellers, and with the underdark being rather too convenient an undead-favouring terrain, I would recommend a strong anti-undead build, if possible with the Sun and Travel Domains (its freedom of movement can be a real lifesaver , plus the domain spells are exceedingly useful for a cleric), Domain Spontanity, Extra Turning (= Extra Mileage), Divine Might plus Power Attack.
You can duplicate Sun Domain's power with the Disciple of the Sun Feat from CD if you want another build with Travel domain, though. A very good prestige class could well be the "Eye of Horus-Re" from PGoF (lawful good, though) - while you loose one casting level, you gain an additional boost to turning because now every turn attempt of yours is "greater" ahem, destructive instead of just turning, you gain darkvision and lowlightvision (and later blind fighting ) for free, your light spells are stronger, and you get bonus domains from among those available for Horus. As powerful an anti-undead PrC is the "Radiant Servant of Pelor" from CD (neutral good as written). Better Fort save, better caster level compared to the Eye of Horus, but less vision based extras and may need rework for the FRs.
As for the weaponry - given that the opposition is going to be Drow and other underdarkish unpleasantness (judging from the title here), go for something with holy (+2), Ghost Touch or Truesilver (both +1, later from Ghostwalk CS) enhancement - which should work equally well against nefarious drows, undead, most aberrations and any demonic assistance for which the drows' are so infamous. Anything else is your choice, but a hefty +3 to attack and damage has never hurt anyone, IMHO. Make the weapon adamantine if you can wiggle it out of the GM - this will kick the teeth on many nasty DRs.
And if you feel you need some short range potential you can solve that by taking the "Gauntlets of Thaarnahm the Vigilant" from PGoF (10000gp) which turn any weapon into a thrown + returning weapon in your hands. Ever thrown a 2-handed holy greatsword at a foe trying to keep out of reach ?
Armour ? Pick what you like and make it "ghost touch" for safety's sake. Get some natural armour item and/or perhaps a ring of protection (though that does not stack well with "shield of faith" )
Get some Darkvision item ( you are going to be underground, so low light will be rather useless ) for yourself and if you feel mean, opt for a ring of the darkhidden (8000gp, MOF) for a cheap way to make yourself invisible if noone has a light on underground. Boots of Tremorsense (A&E if I am not mistaken) could be helpful as well to detect invisible/sneaky opposition.
Get some CHA and WIS boost items - more bonus spells, more and stronger turn attempts, better smiting , better DCs for your own spells.
Aquamarines of Spell Extension (PGoF again) are dirt cheap, and can "extend" any spell up to level 6 once per day - double duration Divine Strength sound good ?
Lesser Metamagic Rod of Silence and Extension maybe useful, depending upon circumstances, especially for their low price.
