The last speaker I would purchase is the center channel depending on the listener position..how far from center you are sitting and the imaging capbilities of your left/right mains. If you sit less than 20 degrees off center and your mains image very well, you can get buy without a center channel. I would get the surrounds, sub ( dependng on how low your mains can play ) and lastly the center channel. Some people claim that the center channel is the most important speaker of the group but I always challenge them with this question...when playing 2 channel music, what speakers are responsible for centering the vocals smack dab in the middle of the soundstage with most recordings?
The only time I would buy a center first even if I met the criteria above is if I know that the speaker model line is end of life and a new model has replaced the current model. Then for the sake of matching the center to teh mains, would I purchase the center channel first.