Both the Miata and Porsche are great cars, used and/or new. However, there’s a huge difference in feel between the two. I mentioned it on another thread here on CC, but I own an original series 1990-1997 Miata. Purist here.
I think the OP’s interest was partially due to the convenience of knowing someone with a BMW for sale. And I apologize for getting the OP a bit off topic. Honda, Toyota and Subaru all have fantastic used sporty options for around $20,000 or less. Knowing BMW’s, the maintenance and depreciation isn’t worth it for me.
About $20,000-ish:
German sporty: Porsche Boxster or Cayman
Japanese sporty: Mazda Miata or Toyota FRS/Subaru BRZ
I’m sure there are a few possibilities that I missed in the Japanese category.