I sent out my std's 10 months in advance just because my friends and family don't generally have a lot of money and wanted to give those who would want to come lots of time to save up. I wanted to actually send std's out a year in advance, but stuff got crazy.
That said, as far as planning goes, I'm sure you coud get it together.
As far as guestst, I would say, expect a lot of them to not be able to make it just because of issues like money or vacation time, but I'm sure you'll still get a great turnout. I think the fact that there's a payment plan definitely helps!
Just let people know ASAP, because remember those that have to get passports need to start making arrangements, Things are a bit slowr a the passport office now! I applied for mine 2 months ago... it's still being processed.