you have indeed missed most everything. d1 really sets up d2 perfectly and there are many subtle references and jokes that i think you will miss out on by NOT playing d1. short answer yes d1 and all dlc are still available for you to purchase. HOWEVER. there isnt a big playerbase and if you wanna raid / strike there is no lfg for that so you needa actually hit people up or just solo the strikes ! you should def get the game tho its great!