I agree with Clive about the 50 f1.8 it's an amazing lens for the money and if good for portraits, Amazon seems to have the best price at the moment -
http://www.camerapricebuster.com/Can...1.8_II_pc.html The Sigma 10-20 is very wide, personally I found it was too much in some situations so the use was quite limited. If possible then try one out, that way you'll be able to get a feel for it and an idea of whether it would be of use to you.
I was looking at studio flash heads last year and did look at the Interfit kit but it looked a bit flimsy and got some dodgy reviews, so I went for an Elinchrom D-Lite kit instead. I've been really pleased with them, they're easy to use and work just as they should. The only problem I've had is all the extra stuff you start 'needing', I've added some backgrounds, a light meter and IR trigger and recently I've bought an extra head to light the background... it can really suck money from you! If you are serious about flash then be warned than some of the prices are on the way up, Elinchrom gear is going up 30% so it's worth looking for someone with stock at the old price.