Looking to buy used Salk speakers? Good luck to you sir.
That said, the matching center for your towers would be Salk VERACITY HT2C.
No need to worry too much about surround/effect speakers. I'd go with cheap ones from NHT for example.
On 2nd hand receivers, I won't recommend anything less than HDMI 2.0 AND HDCP 2.2 or you'll be shooting yourself in the foot from (a near) future proof standpoint.
Lastly, a sub would help, not just with bass, but lower midbass clarity as well.
It doesn't have to brand match, but it should be sized to your room total air volume size and your desire for strong bass, limited by your budget.
Go to brands would be HSU, SVS, Rythmic and PSA.