Markw nailed it down. While I think that you can get do fine without a center speaker, and are better off with no center speaker than a severely mismatched one, the most important consideration in buying a center speaker is the timbre match. And if a speaker line gets discontinued/updated, the new version might not match up as well. Given that reality, the center speaker should be a priority. If not for the need to timbre match the center speaker while it's still available, I would actually put the center speaker behind the surrounds and subwoofer in importance.

The surround speakers are not quite as critical a match, and you can get away with mismatched speakers as a temporary measure if you have some spares lying around. But, once you've timbre matched your surround speakers, there's no turning back.IMO, the split surround effect is what makes modern home theater what it is. Before DD 5.1 came along, you did not have this kind of total envelopment. Use whatever speakers you can,so you can get a hint of what surround's all about.

The timing of the subwoofer to some extent depends on the bass response from your main speakers. The Studio 20s produce a decent amount of bass for their size, but they do not go anywhere near that lower octave that you find with the LFE track on movie soundtracks. In your case, a subwoofer might be a somewhat higher priority. Between the surrounds and the sub, I would go with the surrounds, but that's my own preference. Your priorities might differ.