We just did a F&W day in September as part of an adults only weekend. It was AMAZING fun, but we only focused on enjoying the food and wandering around Epcot. No seminars or shows or anything like that.
RE: the QSDP versus OOP...the food booths use snack credits, you can convert 1 QS to 3 snack credits but they all have to be used at the same booth. While it's convenient I don't necessarily see value in that unless you just want to try one or two booths as part of a longer trip to WDW.
We budgeted $150PP for the day and didn't even hit all of the booths. We did manage to hit a significant number of them, and were fat and happy by the end of the day
We got there at noon and basically ate and drank our way around the festival up until about 20 minutes before IllumiNations. What you would need to spend OOP really depends on what your goals are. $200 is probably a safe number to aim for if you want to try everything (not including any paid demonstrations/classes) as I expect the prices this year will be a little higher than last year.