Minimum you really want is 250 w/s as that is roughly equal to a camera flash with a guide number of 45.
150w/s is probably closer to guide number 30 so you would be limited to a fairly wide aperture.
Remember diffusion whether of constant light or flash absorbs output. A bounce brolly will loose you one stop minimum over direct lighting.
I know you are starting off thinking portraits but once you have the kit its surprising how quickly you will want to try something else so these are important considerations.
Lucky out time now
( Lucky is the budgie and he shouts if he feels left out
) , so I will look through WHE around 7:00pm if you give an idea of total budget.
Catch you later.
Don