Question for macbooks

Nobody can possibly know what's best for you other than you. If you want to save a significant amount of money on a Mac, shop Apple's Certified Refurbished products page.

Regarding used Macs, I strongly encourage you to consider lobsterghost1's User Tip The All Too Common SAD Reality of Buying a Used iPad/iPhone - Apple Community. Despite its title, nearly all of it is equally applicable to Macs.

On that subject,

Just out of curiosity, why do you want to run Catalina?

It won't run 32-bit applications – and it's also too old to be one of the "most recent three" (Monterey, Ventura, Sonoma) that get the most "love" from developers like Adobe and Microsoft.