The 40mm should do it for street photography.
What are the kit lenses? I'm only aware of the 24-105mmL being offered as a kit lens in Canada and the USA, but it can be different depending on which country you are in.
The kit lenes 24-105mml
But for modeling
in a studio the 24-105 is actually excellent since your will typically be shooting at f8 to f11 ish
at f8 this lens is really really sharp
also same deal with your 50mm stop it down there and shoot in a studio and its gonna be very very sharp
depending on your budget consider an 85mm either the canon f1.8 if you are on a budget around $350 ish
or the sigma 85 f1.4 if you have more to spend around $800
but I would suggest getting some strobes first and using the 24-105 while you get the hang of it all
I recommend this setuphttp://www.bhphotovideo.com/c/product/906651-REG/elinchrom_el_20852kit_d_lite_rx_4_400w.html
then look on ebay for a cheap backdrop setup
gets you started quite cheaply