Blu-Ray drive - how to choose one?

My friend is thinking of buying a Blu-Ray drive and asked me for help. However I'm running an old CD-ROM and have no idea about the newer writing technologies...
So how should I choose one? I see there is a cache - 4mb/8mb. Is it an important thing, and how is it important (what it does)? I assume that it buffers writing/reading memory, but don't know for sure.
His budged is unlimited, so I guess it could be a 60$ or 600$ drive, though I'd prefer an explanation rather than a link, since I'm trying to catch up with the new technologies.

Im using a "cheap" LG 12x BD Drive. I paid ~$100 for it a year or so ago, but you can find them now for under $60. Ive had no issues with it.

Main thing with BluRay is the player you use. Ive been using Power DVD since I bought it and it works well.

In terms of optical drives, I would just get whatever's cheapest. Some of the cheap ones advertise as a combo drive but can't burn DVDs, a true BD Combo Drive is a DVD Burner + Bluray Reader.