Vodka is a must have, its neutral taste will let you mix it with cranberry juice, oj, punch, 7up, tonic, whatever your guests like. Buy some dry vermouth and you could make vodka martinis (the half bottle is like $5 and should be all you need). Add roses sweetened lime juice and you have a vodka gimlet. Rum and coke or dr pepper is also a popular combination. Buy some ginger beer and lime juice and you could make dark and stormys. Add lime juice and simple syrup and you have a daiquiri. A little more complicated and you can make mojitos. I love gin but its one of the least popular liquors so if you're on a budget, I wouldn't buy it. Whiskey is a good choice too, lots of people like it with coke, on the rocks, or neat. Add a bottle of sweet vermouth and some bitters and you can make an approximation of a manhattan. You could get tequila, triple sec, and lime juice and make margaritas on the rocks. Get some frozen limeade and you could make frozen margaritas too.
Overall I'd go with vodka as a positive and then try to get something else based on any knowledge you have on what your friends usually drink.
Overall I'd go with vodka as a positive and then try to get something else based on any knowledge you have on what your friends usually drink.