Incandescent light === bullshit.

It doesn't matter how many watts of it you have it's still shit for growing stuff. They get way too hot and they're the wrong spectrum. The best lights for starting plants (in my opinion, and many other people's opinion) are fluorescents. They have a good spectrum for plant growth, they don't get too hot, they're cheap to run and pretty cheap to buy. Your 400w HPS will obviously be far better than the incandescents you're currently using but you don't need 400w to start young plants, it just seems like a waste of power (if you're concerned about power consumption). I would get hold of some fluorescents to start seedlings/ root cuttings/ keep mother plants and use your HPS for flowering only; this way you can have a continuous growth system---
buds come out, clones go in, buds come out, clones go in etc, etc.

Soil- regular compost from the garden center works fine, I use it, always have. It's cheap and you don't have to adjust the pH or anything. It will support young plants for about 3 or 4 weeks before you need to fertilize. I get a huge compacted bag for just over a fiver, it's never let me down- touch wood. I mix it with perlite and vermiculite or sometimes just perlite.
Fertilizer- I've used Miracle-Gro (15/30/15) in the past but it always seems to burn the plants no matter how careful you are with it (it's a chemical feed). This time round I've gone for the organic feed BIO-BIZZ. You get Bio-Bizz-Grow for vegetative growth and Bio-Bizz-Bloom for flowering. I got this from http://www.growell.co.uk

I wish you all the best,

catch ya later