Need advice in buying a new ps3

Hi. im planning to buy a new ps3 and would appreciate if you could answer this few questions:

1. What is the ps3 version that is currently HACKABLE and what is the highest version DOWNGRADABLE?

2. Is there a way to know if a ps3 is hackable by just looking at the serial on the box and not opening and looking its FW in system software? Because someone is selling me one, and dont know what FW is it. He doesnt want to open its box for brand new selling purposes.

Thanks.:rock: