I tend to buy balls as and when I need them, for instance:
If a ball has "died" I replace it
If I come across a condition which requires different equipment, and I think I'm likely to encounter it again, I'll buy a ball for that type of shot.
Because I don't buy balls very often, I try and give them quite generic and smooth drillings, as I can't afford to have anything to be too condition specific.
Before I got injured, I was bowling at a reasonable competitive level, so I don't think it's entirely accurate to say that you can't be bowling competitively buying 1-2 balls per year. I've got about 7 or so at the moment, bought between January this year and January 05.
I would say the best thing to do if you're not very knowledgable is to find a good pro shop operator and place yourself in his/her hands, tell them what you need the ball for and how much you want to spend and they should be able to sort you out.
If you want to learn more, by all means do, when I'm looking at getting a new ball, I have more of a dialogue with my pro about what to get and which drilling but I still place a lot of trust in his judgement, it's what I'm paying him for after all...
--------------------
Reporting from England