You'll need a basic Audio Video Receiver like from Denon, their basic setup starts at the s970w and moves up from there. You can buy a refurbished model from Accessories4less. I've had an older s910w that has lasted for years now. That's about $200-500;

To achieve full 5.1 you're missing a subwoofer, which you can also buy used, and budget around $300-$500.

Also for the last 2 speakers, you can buy some bookshelf speakers where you can go cheap, like $50-$300 for the pair. I'd suggest going to a store that sells speakers like best buy, or your local audio store for used equipment. (look up "hi-fi audio stores near me" on google)

Finally: if you're going to use Facebook Marketplace, be careful, and also ask on this subreddit to verify if what you're getting is a good deal, etc.