Little help with a DM setting (Bet I'm the first person to use that title)

[TrueJordan]

So I have a bit of the campaign, but I need a bit of help for the rest.
Some information of what I have first:
-Only using the SRD, sans psionics and with ninjas (cuz ninjas
)
- The players are a 7th level fighter, 8th level paladin, 9th level rouge with a 7th level lycanthropic wizard cohort
-The game will last around 5-6 hours depending on how things work out, and I'd like for this to neatly wrap up in that session. Also it's the first time I'm DMing in a group where we switch off and it's my turn this week.
-I want them to sneak into a monastery, to try and find a 'major magical item' since the Bigcity Paladin Force (BPF) are pretty strict and don't allow any non-declared magical items into the town, especially powerful ones. Our group has helped the paladins in the past, so it's plausible that they'll help out again. I'm planning to make them fight basically vanilla monks and ninjas in there, because I like their fighting style and apparently they can train together so why not. The magical item will be the Mirror of Opposition (http://www.d20srd.org/srd/magicItems/wondrousItems.htm#mirrorofOpposition) and eventually they'll have to fight themselves for the last fight.
I plan for that to be the treasure as well, even if it's a little on the expensive side.

Here's what I need:
How will I do this in a way that won't have every room be a new, fighting room? I want there to be a sense of infiltration, but it won't be fun if they're just rolling their 'move silently' rolls against everyone else's 'listen' checks all the time.
Additionally, how can I make all the characters feel necessary? Everyone's basically standard (Skillmonkey and efficient rouge, powerful wizard, strong fighter, charismatic paladin).

Thanks so much in advance. Any other advice would be awesome as well.

[roguemetal]

Detail the spaces being used. Creating in depth senses of the scenery will allow players to be fully aware of their situation, and give them choices so that they aren't just sneaking all the way there. Include traps and things that don't normally detect as traps (such as some mud on the floor which guards could track if the players step in it).

Add options for taking obvious shortcuts that seem potentially good but risky.

Create a time limit for a suspenseful environment (you don't even actually need to honor the time limit, just the idea of it can get players feeling the suspense.)

Provide impasses, such as a group of Paladins returning at an inopportune time, forcing players to withdraw somewhere or be discovered.

[TrueJordan]

Detail the spaces being used. Creating in depth senses of the scenery will allow players to be fully aware of their situation, and give them choices so that they aren't just sneaking all the way there. Include traps and things that don't normally detect as traps (such as some mud on the floor which guards could track if the players step in it).

Add options for taking obvious shortcuts that seem potentially good but risky.

Create a time limit for a suspenseful environment (you don't even actually need to honor the time limit, just the idea of it can get players feeling the suspense.)

Provide impasses, such as a group of Paladins returning at an inopportune time, forcing players to withdraw somewhere or be discovered.

Yeah, those seem like good ideas, thanks bro.