Epic Bank Heist advice

[Ryze Kuja]

I'm GMing an Evil campaign and our lvl 4 party of five Gestalt PCs have been hired by a mysterious benefactor to perform a bank heist. The party has been informed that there is a safety deposit box in the vault of a bank which is owned by Richdude McMoneybags and the entire Moneybags family was lost at sea some several years ago, and no one is quite sure what's in this safety deposit box but it can be easily assumed that there will be treasure, maps to other treasure, keys to his many mansions/chateaus, etc.

Anywho, I want this Bank Heist to be really really fun and iconic for the players. Currently, the bank is two stories, and the vault "floats" above the bank. The only way in or out of the floating vault is through a teleportation circle that is well-guarded on the 2nd floor of the bank, and the vault is afforded every imaginable protection against magic and psionics. Also, there is a sewer system that runs underneath the street where the bank is located.

Here's my question/advice needed:

What kind of challenges and "alternate avenues" can I add to this to make it both tough and fun at the same time without making it a smash n grab encounter?

The party is a Spy/Alchemist (Infiltrator), Psion/SynthesistSummoner (think Venom, from Spiderman), Cavalier/Druid (Prize Fighter), Oracle/Witch(Necromancer), and Witch/Bard (Seducer).

I'm thinking there should be a Rogue-ish NPC who has been casing the bank for his own heist and can give the party some assistance, or possibly one of the bankers can be blackmailed with a drinking/gambling problem...

[Slim Jim]

There will be Scrying Eyes for security, and paladins qua "Pinkertons" to hunt down thieves after the culprits are identified.

[Kayerloth]

1) After the PCs have been casing the bank for a bit and start planning they learn the bank plans to rotate the security arrangements. New traps, new passwords, new ... stuff in X number of days.
2) The PCs learn of a second group looking into pulling a heist. 2a) The second group really are looking into pulling off a heist or 2b) The second group was actually hired to look into the security on the bank and are testing the defenses ... and will definitely attempt to warn their employers if they learn about the PCs group.
3) Your Rogue-ish npc is actually a foreign agent who has learned of some 'material' held in the bank. He'll be happy to help but will also be looking to make sure the PCs get caught while he gets away/vanishes with the 'material'
4) One of your PCs has a contact who tips them off to the fact that some 'material' is kept in the bank (think The Bank Job, Jason Statham movie)
5) The mysterious 'benefactor' turns up dead in their room. 5a) before they complete the heist or 5b) after the heist but before they get (fully?) paid.
6) The party becomes aware they are being watched, but by who!? Is it members of the countries counter-intelligence (looking for the above agent), law enforcement or rival of the 'benefactor'