You can do straight knitting on circulars. The only difference is that when you come to the end of the row, you don't join. You just turn the work around like you would on straight needles. I would still keep your straight needles, though.
For common sizes of DPN's, that's a tough one for me because I see patterns that call for all different sizes. I think it also depends on what it is you're going to knit with them. If you plan on knitting a lot of socks, I'd go with a smaller size. For circulars, I personally prefer bamboo, but I hear a lot of people here just rave about Knit Picks Options. I've never used the KnitPicks Options, so I can't really give my opinion on them (although Christmas is coming and I may be hinting to DH to get me a set of Options
). I had some of the Boye Needlemaster Interchangeable Needles, but I didn't care for them. If you're looking for a common length, I'd go with the 29 inch length first.
You might want a cable needle if you feel like you want to try to knit cables.
You don't have to get stitch holders. You can thread a bit of waste yarn through your stitches to hold them and keep them live.