I would go for a pure singleplayer experience because otherwise it's almost the same as developing two different games (and for said reasons concerning multiplayer listed in your post).

(Seemingly) intelligent AI surely will be difficult but I find that interesting as well. Additionally I think not all enemy types have to be clever, one may add stupid cannon fodder to highlight more clever enemies.
I have a story in mind which is simple yet functional, the thing that is lacking though is my level design skill and the patience to create more than a few rooms... and no matter how good the gameplay feels, this is what makes or breaks such a game.

Btw. I've once (about a year ago) tried to make this game (idea) into a 3rd person game, yet I found that to be even more difficult (in terms of good shooting mechanics, player movement and the like).