one thing to remember when putting out that cash for needles though is that you will be able to use them over and over. so while it may seem expensive at first, over the lifetime of knitting the cost does actually go down. one thing you might want to consider investing in is a set of interchangeable needles. again...may seem like a large outlay of cash but with so many varieties of needle sizes and lengths the cost definitely does go down.
as far as your socks...you can get longer DPNs...maybe a 7inch will work better. or you can try knitting on two circs too. or magic loop. it all depends on what your preferred method would be. you can get 7inch DPNs at hobby lobby for about $2-3 if that is what you want to use.
i am very curious about the sock pattern that calls for size 13 dpns though. what kind of yarn does it take?