I would have the speeches/gift giving before you serve the food- some people may leave as soon as they have eaten and once the speeches are over everyone can relax and enjoy the dinner. Plus it would be embarrassing if the speakers/retiree had accidentally spilled food on themselves while eating (presumably you're taking photos :-) ).
I would put something like "to assist with catering, please let us know if you are/are not coming" on the invitation- do you have some kind of work email people can respond to easily? And follow up those who haven't responded in plenty of time to get the catering numbers right. (Throw into the office gossip pool the info that those who don't respond may find there isn't any food for them ;-) .)
Does your Christmas party include dinner? You may find more people turn up due to the free meal, and the fact that there won't be heaps of other retirement parties/other social engagements filling up the calendar as there are at Christmas.

Thanks Colleen. As I said, I have planned our company Chrismas party, but never a retriement party. Your suggestions are helpful and I will follow your advise. An additional question. Our company policy states that we will not pay for alcohol, but is it okay to have a cash beer and wine bar? If so, it will state so on the invite. Everyone else thinks it is okay, but I am reluctant to oblige. I'm not sure if it is the place for alcohol. Yes, the guest of honor has been known to enjoy a glass of wine.

Can't advise on the alcohol. Where I live it would be illegal as alcohol can only be sold on licenced premises or where a special event licence has been granted, and never in a public park.
I'm with you, not sure it's the place for alcohol, and it's a little early. (I'd add that I drink very little). Perhaps if it were permitted then a very limited bar, beer and wine only, and servers instructed not to serve anyone past the "slightly squiffy" stage- the last thing you want is for everyone to remember this party because of some memorable drunken faux pas.