Ideally you should use a halide for veg and a sodium for flowering, but there's people around that do whole grows with just one light. I'm not sure if it matters which one you use, i've used sodium from start to finish and i've seen others using mh, neither seems to have a big advantage over the other. I'd get a couple of cfls for the first couple weeks when your plants are fragile tho, (2 cfls, sockets and plugs will only cost around £10) you can use mh or sodium but you'll need to judge the distance you have them from your plants to prevent them from getting fried.