I don't really agree with the idea of having more than 1 or possibly 2 PCs in such a campaign. If you went ahead with the team of specialists approach, each one with his own niche (face, sniper, insider, sneak) you'd simply end up running four inter-connected solo campaigns; probably more than you'd care to handle. What I mean to say is, the party will rarely be together except to discuss a contract, plan missions or split up the spoils afterwards.
In such a campaign you'd end up running the insider by himself through his attempts to infiltrate a venue, all the while running the face through his attempts to negotiate contracts and speak with contacts on the street, all the while running the sniper as he moves into position in a nearby clocktower and waits patiently for his target to present himself. There's little reason (except for sheer masochism) to shoe-horn these people into a "party" since they'll all be operating individually most of the time anyway.
Keep it simple and stick with one PC assassin; you'll save yourself a lot of extra work. You could go ahead and have two PCs if you like, but they'd likely have to have similar stealth abilities so that they can stick together as they make their way towards their joint target. Any more than two sneaky types and the campaign starts getting silly;
Q: How many ninjas does it take to stab one guy?
A: One to hold the knife and two to manoeuvre the target onto the blade.
There's nothing wrong (and many right things) about placing such a campaign in the Forgotten Realms. Waterdeep, for example, would be a fine place for such back-alley assassinations. And don't worry about clerics, raise deads and magic in general; they're what would make the game interesting. Check out Steven Brust's Vlad Taltos novels if you want to see how assassination might work in a high-magic fantasy setting. In the world in which the novels are set most everyone can communicate telepathically and teleport and raising the dead is very easy. In the novels the protagonist assassin character is often contracted to kill people, simply as a scare-tactic. There are few things more terrifying that waking up from death... Permanent death is a little trickier, more expensive and usually reserved for those individuals who really pissed somebody off. For such contracts, the protagonist usually makes use of various magics to make the death stick.
The possibility of raise dead being cast is easy enough to circumvent; simply mutilate the body in such a way that it is no longer "whole" and so is ineligible for use with the 5th level version. To circumvent the possibility of resurrection, take the body with you and dispose of it carefully later; no body, no resurrection. True resurrection is tough to beat, but hardly impossible. If a target is worth that much to kill, then your employer better be ready to shell-out enough to cash to cover the cost of the soul-trapping/destroying magics that'll be required for the job. Also, keep in mind the amounts of money required to bring someone back from the dead are excessive; they're not easily attainable for most people, and its not everyone who is going to be willing to dump that much cash on an ex-associate that they might be better off without.
There are plenty of possibilities to explore in such a game.
![:]](http://www.enworld.org/forum/images/smilies/devious.png "Devious :]")
Also, corruption helps immensely.
