These:
I know I say it often, but if you're heading down the road of 30k, plan your theme and army. Legions can be all things, but there is a huge amount of options available for a typical 30k army. Getting those two books before anything else will help you avoid buying units that you end up never using.
Though, saying that, do you have any ideas on how you want to play your Death Guard? A few of us can point in the direction of certain units that play to the style you'd like to go down