Buy everything used, but with some warranty if possible.

Most CPUs are very expensive compared to how much they can mine. I'm a fan of buying cheap processors and not seeing it as time waste(since it's very cheap).

For my picks on CPU and Mobos, take a look here:
https://bitcoinminingsoftware2019.com/bitcoin-mining-beginners-guide/best-bitcoin-mining-hardware/

For my picks on GPUs, take a look here:
https://bitcoinminingsoftware2019.com/bitcoin-mining-with-gpu-top-choices-for-2019/

The numbers change with time and price changes, but should give you some overview.

On more up to date numbers, take a look here:
https://www.kryptex.org/en/best-gpus-for-mining

I like that the last table can be sorted by profitability, payback and even various coins.