If you're looking for versatility... Centennial is the ticket. I rarely leave it out if an IPA. I also saw a list a couple years ago and cents were the most widely used hop in commercial IPAs. As far as the other or goes, start with Simcoe, but you can't go wrong with chinook or Columbus as IPA bittering hops. Amarillo, citra, galaxy, mosaic, cascade.... All good fir ipas.
I find it more of a bargain to buy pounds to half pounds. But be we warned, if you start buying in bulk, prepare to have your freezer full if hops within a year lol