I play a TON of EDH and also run a Grim-grin build. Here is mine: http://deckbox.org/sets/139813
I know the current version is different than what I have listed here by a few cards but, in general, this is about as polished of a list as you'll find out there and I strongly recommend picking up every card there if you decide you're serious about the deck.
Here is a link to a previous discussion I had on this same topic; the last entry is a fairly up-to-date breakdown on what cards you should focus on getting and how important they are to the strategy overall.
http://deckbox.org/forum/viewtopic.php?id=1465
EDH deck breakdowns are massive topics so let's start there and see if that answers most of your questions.