Best alcohol for party?

Good responses above, but an important factor that hasn't been addressed, which will influence people's drink selections, is the kind of food that is being served. It would be helpful if you could give us some idea of that.

Also, provided that it is a suitable match to the food, IME the most popular drink selection among the type of crowd you appear to be describing is white wine, so I'd purchase a goodly amount of that. There are lots of good choices, but you most likely can't go wrong with any French white under the Jadot label that fits within your budget, Jadot being a négociant.

Regards,
-- Al

Sounds like a plan!

White wine would most likely be fine with all of that except for some kinds of sausages, certain kinds being better with red wine or beer. Among white wines, besides the ones I and others mentioned earlier you might consider a dry German Riesling, to go with the sausages in particular.

Champagne would probably be fine with everything, although a downside of champagne is that to do it right you would want to supply the proper kind of glasses (or plastic equivalents), and dealing with and setting out an adequate quantity of them may be a bit cumbersome. The proper kind of glass is a tall, thin flute, such as what is shown in the first photo here, NOT a "coupe" style glass, shown in the second photo.

I would also provide bottled water, as others mentioned earlier, especially Perrier sparkling water, and a quantity of beer.

Assuming you omit the hard liquor that has been mentioned, and stick primarily to wine and/or champagne, for 20 or so moderate drinkers I would assume, to be safe, an average of two drink-equivalents of wine or champagne per person (assuming 5 ounces of wine per drink-equivalent) plus some margin. That adds up to about 8 bottles of wine or champagne, plus margin. Personally, I'd get about 10 bottles and enjoy what is left over with my wife, during the following days :-)

If you allocate that between red and white, as opposed to champagne, make the majority of it white, as I suggested in my earlier post.

Regards,
-- Al