For meaning (b), we would say, “I was not fool enough to accept their offer,” “I was not enough of a fool to accept their offer,” or, “I was not stupid enough to accept their offer.”
Interesting. By the looks of it, ‘I was not stupid/a fool’ alone would stand for ‘I did accept it’, while ‘I was not stupid/ fool enough’ would mean otherwise. If so, ‘I was not so stupid as to take…’ should equal ‘It was wise to not take…’ and ‘I was not so stupid as not to take…’–‘It was wise to take…’
• Also, I wonder if there’s any need in Perfect construction ‘I was not stupid/a fool to’ve taken’ (the action did happen in the past).