Get as much Death Ward and Protection from / Magic Circle Against Evil effects/scrolls as you can. I don't know how much money you can ask for, so I'll just throw out a lot of ideas.
Phylactery of Undead Turning is expensive, but would be very good for the Cleric. If this is going to be a one-day strike, there is no reason for the cleric not to make use of Incense of Meditation.
Other than that, I assume you're familiar with DMG items, so I'll point out some from Magic Item Compendium.
For armor crystals, a crystal of lifekeeping would be helpful in lieu of Death Ward. As a sneaky scout type, you may want Bane Blind enhancement (cost of +15,000 gp to armor) keyed to Undead, which would let you 3/day be undetectable by them for 10 rounds or until you attack. It prevents blindsense, tremorsense, scent, etc... Unfortunately, it only foils nonmagical senses.
Truedeath weapon crystals may be a good idea, as is the Sacred weapon property (+1; +1d6 damage to undead or +2d6 to evil outsiders; weapon is good-aligned). If you have ghost touch on your weapon already, you can upgrade it to Ghost Strike for another +1 modifier, which lets you SA and crit undead. The greater version of truedeath crystal (10,000 gp, only works on +3 or better weapon) also lets you SA (but not crit) undead. Not sure either is worth it, even if you have Assassin's Stance.
Goggles of Lifesight (p. 108) let you figure out if something is alive, undead, or neither.
Ring of the Darkhidden (p. 122) is cheap and hides you from darkvision. I believe all undead have darkvision, and if it is dark (likely, considering most undead-infested lands tropes), this will make being sneaky all the easier.
Sepulchral Vest (p. 133) gives +5 on saves against undead-generated effects 3/day, and is cheap.
Reliquary Holy Symbol (p. 120) is cheap and could give the cleric more uses of Turn Undead, but takes up the neck slot (ie, the +wis slot

). Could always pay double for a non-body slot... Of course, if the DM allows Night sticks or whatever they're called from Libris Mortis, not much point in getting this.
For item sets, if possible anything from the Vestments of Divinity set would help, though some of the items in the set are only useful to clerics and paladins. The Wraith's Woe set would be useful to anyone in the party. The Vest of Free Movement from the Fleet Warrior's Array is the cheapest means of Freedom of Movement I know of, other than potion, wand, or scroll. At 12,000 gp it's still probably too much, even if the DM lets you request item set pieces on demand.