Congrats;
This is a great hunt! The park is approx 80,000 acres and holds about 1200 elk in Sept. The elk are bugling and you will see lots of elk. Use your deer gun (.270 or larger) with good bullets. Elk are so much hardier than deer. It will possibly be warm in Sept but could rain up there. I was there in Oct for a cow hunt and it was bitter cold and I was there last year for a late season archery hunt and it was in the mid 60's for most of the 11 days that I hunted. Get in good shape! Be ready to get the elk out, they are very liberal about retrieval once you have one down but you have to get the hide off soon. The elk are everywhere. The burn is very popular to hunt bc/ you can see a long way. It is hard to get lost but Nat Geographic makes a good waterproof topo of the area that is my favorite and will help w/ your hunt. Have good broken in hunting boots and be ready to hoof it. I would spend a lot of time scouting this summer.
Good luck.

Brad