Lello,
Never done portraits either, but here is my two penny worth.
I guess the cheapest option is as Mike suggests.
Next up is Duncans as you have the flash units but will still require stands and brollys.
You will have to make a fair bit of use of it to justify a studio setup and in IMO would be better putting that cash into your body or lens piggy bank.
Softboxes are a bit of a problem for camera flash units. Not sure how easy it is nowadays to get hold of an "Apollo" softbox like mine today. Most currently available units are designed to attatch to the front of studio units by way of a mounting ring, so the flash head supports the softbox.
This link should give some idea of what I mean as far as the "Apollo" goes.
http://www.firelily.com/photography/softbox.html
To use a brolly you will need the parts shown in the link less the softbox.
Don