I'd just like to repeat the advice given earlier in this thread, because I think it's damn good advice.
Tell your players that you have concerns about this party's ability to survive, but that you're willing to give it a go if they are. For a player, there's a world of difference between going into an encounter knowing that your own choices mean you've stacked the odds against you, and blindly walking into a TPK.
If the players are aware of their party's weaknesses, then they'e more likely to try to play smart and compensate for those weaknesses.
If you're really worried, I recommend that you start with a couple of encounters which are sparring matches or something - no treasure (because if there was it'd really suck to lose), but both sides are fighting to subdue, not kill. Have the opposing team include the exact mix you're most worried about, and see how the party fare. That way, if the PCs lose they don't die and the adventure continues, but the players will now be aware of the problems their group mix will cause.