I ended up buying a set of interchangable needles because I got all the sizes in the set and it's less expensive than buying a different set of needles in each length. I use them for knitting flat also. As for which ones and quality, it comes down to what you like and are comfortable with. I have Knit Picks Sunstruck needles, made of wood. Others prefer metal or even acrylic. So...it all depends. It's really a personal choice.
I should never overestimate my basic arithmetic skills.