The Sierra 2ex are supposed to be very solid speakers. If you are going for a system that you might never have to upgrade from, those would be a good start for your mains. When you can afford the next step, you could double down on those or buy the Sierra Towers.
Completely agree with Shady, get the best 3.1 system you can afford. You want those to match across the front.
Surrounds can be added later and they do not need to match the mains.
The other speakers Shady mentioned are solid and worth considering, too.
Cheers!