I would go even though I would find it a little difficult. I think that I would just try to remember that even though the company laid you off, it doesn't meen that your colleagues didn't value you and that lunch is a kind gesture. I would also advise to leave enough room for dessert, nothing worse than being to full to stuff down a delicious dessert
Hehehehe. You are moving on and upwards, maybe a chance to move on to something bigger and better. You could look on this as a positive new start, although positive new starts are not always easy!! Damn those bills. Good luck with everything!