In a stealth-oriented FPS ? Not sure I've played one. I've played several third person stealth games, though. Metal Gear series and Splinter Cell of course. Typically what makes me enjoy a campaign is the story; the levels need only fit the story, and put you in places that make sense to advance the story. Any side missions need to have their own story that makes sense with the main story.
The length of the levels themselves don't matter, but make sure you play through them several times to make sure you've spaced the save points or triggers or auto-saves or checkpoints or whatever, in the appropriate spots. In these types of games, players typically want to be able to quit whenever and not lose too much progress.
This is definitely a big undertaking, but I applaud you for attempting this. It's the first steps that eventually lead to great indie games. I, for one, have never though of budget as a true constraint. I think a great game can be made from sheer willpower and a few people with a shared vision (this happens quite a lot in the indie scene, actually). But I will say, you have to be careful what you decide to splurge on and what you're more conservative with. For example, as a smaller team, I would suggest cutting back in the graphics department in order to ensure the gameplay itself gets the most attention. Don't hire voice actors, too expensive. Instead, use the old-school RPG method - text. Spend time on the level design, but reuse art assets wherever you can. Nobody's going to care that they've seen the same dumpster model in one level on another, especially in an indie game. These are just a few suggestions, but you get the idea.
Good luck, man. Hope to see a completed product one day.
