In addition to trains, I'd suggest looking at being close to major bus routes. Unfortunately, the practical reality right now is that Brisbane is more of a bus-based city than a train-based one, and so ruling out buses when considering where to live limits you dramatically.

But don't just look at if there's any bus nearby. Look for one of the major "BUZ" routes. Or even better, one of the new Metro routes (or potentially the proposed future Metro routes). Anywhere on the South-Eastern Busway is likely to be good, as is the Northern Busway. To the North, I'd say anywhere as far out as Chermside (but not much further) would be viable if you're within a good distance of a stop used by the 330, 333, or 340. Beware the 370 though, which is a much less frequent and slower service.

Consider your needs outside of just commuting to the city. Where is your social life? Will you be able to get to friends' houses, sporting clubs, and other social activities you enjoy from where you live, at the times you'll likely be doing it? Are there shops within walking or safe cycling distance? These are usually the hardest problems to solve.