Depending on the music your playing will determine your pickup choice. I would go with a Strat Hot Rails ( seymour duncan ) for the bridge ( VERY chunky and responsive, I have one in my Tele. It is in fact a humbucker that fits into a single coil slot. I would recommend splitting it so you can get single/humbucker sound out of it.)
Neck go for a jazz pickup maybe? Low response, good for acoustic type stuff. But the problem is, it might not sound good for chunky rythm on the neck and I don't know how it will sound with overdrive.
The middle pickup is a gamble. I'd leave that one alone until you really know what you want.
If I may say, on a personal note, I would not recommend buying an American Strat, especially if your changing pickups. In my experience ( and many others on here ) the mexican Fenders are just as good as the Americans. I would say, buy a mexican strat and then do all this stuff to it. It is very possible to make it do everything you want. I'd try to find one with a locking tremolo. Then change out the pickups to what you want. If you are getting humbucking pickups than I'd split them. For this you need to change your tone and or volume pots ( up to you ) or you can use toggle switches.
Electric Guitars are the inspiration for cries of "Turn that damn thing down"-Gibson website