Only get DI if you're wearing DL. If you do have DL already, may as well get the DI, its a nice combo and good on any class. I'd go with this one for versatility's sake and because it's good no matter what you're doing. (If you don't have a Dark Lord footgear then get that first.)
BK is alright, useful if you're leveling with a weapon or by demo mobbing. But I'd pick DL+DI for demo mobbing myself unless you prefer to wear Eddga, then BK would be fine I guess.
Cast time is not a major concern for alchemist, all the skills are pretty quick cast to begin with, just get a phen or BB acessory.
