I would get an extra set of needles for two reasons.
First being that's it's damn tricky putting stitches onto stitch holders or straight needles to hold them when you get to the crotch bit. You would need three per leg (plus the ones that hold the crotch till you graft it...and you'd have needles sticking out all over the place while you try to knit. I'm speaking from personal experience here too
Also if you get one circ then if you get 40cm's they are too long to do the legs and you risk stretching the knitting too far or 30cm's then it get's a bit crowded on the needle for a larger size and you can drop stitches easier.
I started off getting 1x40cm and 1x30cm but then got another 30cm plus DPN's to make it easier on myself. It seems a bit I suppose but I also knit beanies with them too.