you can install 3.55 cfw over 3.55 ofw 100%
i'm not sure about the 80gb consoles being bc for ps2 as im pretty sure they werent released in my country. if you can find out the model number eg cecha01 i can tell you then. but as far as i know cfw doesnt affect the bc so if it came with bc originally it will be ok.
just be careful buying a fat ps3 as the release consoles are poorly designed and have a YLOD fault. make sure the security sticker is still attached where you pull the console apart to ensure it hasnt been repaired already.