I wouldn't limit yourself that much mate, once you get a sturdy collection behind you then it won't be an issue.

I play dnb exclusively too and the only keys I find myself buying very little of is b minor but there is so much deep techy dnb you can play off key without an issue (eg. stripped back skeptical tunes can be played off key and it rarely sounds bad).

Imo don't even look at the keys when you're buying it and enjoy the fun of finding out what goes with what when you're having a jam.

edit - btw 99% of dnb tunes seem to be in minor keys (well a quick browse of my collection seems to reflect that anyway)
http://soundcloud.com/hifire

Catch me playing @

Gangarang @ Rumpus Room 8th of June
Junkyard Sessions @ Rumpus Room 16th of June