Well i'm also looking in to buying my first studio kit and have a budget of £1000 along with buying a new lens, external flash and a few other bits and bobs and so on my very limited budget I've been thinking about getting this [link] I've thought about getting slightly higher powered ones but to be honest I don't think 3x 300w is a bad setup at all for my purposes
400 is more than enough. When I was buying my first lights I had the same fear as you and ended up going with a pair of 500w/s lights. I have never once had them over about 40% power. Usually I shoot them at about 20% and there are many times that I find myself annoyed because I can't shoot wide open because even at their lowest setting my subject ends up over-exposed.
Have you tried gridding the lights or gelling them with an ND film? Taping foam core to the front of your soft boxes to turn them into strip lights? Adding an extra scrim? Flipping them around and bouncing them off sheets of grey foamcore? I'm guessing that you don’t necessarily want to move them farther away from your subject because you prefer a more dramatic falloff, but there are lots of ways to kill the power in your lights.
You could get away with two 400-w/s heads, sure. Three would be even better. That’d give you 1200 total watt-seconds to play around with – way more than enough for most applications, especially if you’re mixing in ambient. Beyond the raw power, though, having three heads gives you some flexibility in arranging your lighting setups.
Are you talking about monolights? Not that it makes a difference, but you kinda threw me for a second when you mentioned an extra head.
As ~Rcooper suggested, 400 w/s for each of your lights is going to be way more than you need if your studio is moderately small, unless you prefer shooting people at f/11 or f/16, but someday you may need that much. Your modifier list sounds like a good place to start. My gut feeling about the third light would be to get one that’s the same power or even less, rather than more. Save a few bucks and keep it on hand as an emergency fund. The one thing I’ve found about gear is that there’s almost always something that you just can’t do without, but unless you’re using it a lot, it’s better to rent. I do that with lenses, secondary lighting packs and (until recently) backdrop kits.