Don't over think it. Everyone always makes these big, long, expensive lists of must haves.
If it's in your budget- that's awesome. New toys (especially quality ones) are always fun. But not necessarily needed.
All you really need is a good pair of boots that you can hike in, your tag, your firearm, a sharp knife, a backpack, and plenty of water/calories to fuel yourself.
There are lots of things out there (binoculars, game bags, a pack frame, more comfortable equipment and clothing, the list can be long....) That make the hunt easier and more enjoyable. But purchase or borrow whatever you can, and don't worry about the rest. Get out there, enjoy your hunt, and learn from every experience.
If your buddy is an experienced elk hunter theres a good chance he has allot of the stuff you'll need already and may be able to help you out by letting you use some of it.
Keep us updated on the hunt and how everything is going.
-Jake