Where are you located? Just wondering as I may be able to point you in the right direction for retailers who specialise in catering for these type of events.
I would average consumption to 5-6 drinks per person beer and wine combined. Some may have many more than this and some may drink very little or non at all.
I would stick with 4-5 varietals-
Sauvignon Blanc or Pinot Gris/Grigio
Shiraz or Shiraz Cabernet
And a small amount of Chardonnay and Pinot Noir.
I found that purchasing wine over time spread the cost & saved me $$. For example, I saw NV Chandon at first choice for $18 & swooped.