From this site, it says that Salt Creek flows Northwest of Valparaiso and, via the Little Calumet, eventually finds it way into Lake Michigan.
Fly Fishing the Salt Creek in Indiana - Fly Fishing Community
The species that are given in various sites that pop up when you search <Salt Creek, IN, Fly Fishing> are "Salmon", "Steelhead" and "Trout". That gives you a pretty big variation in the gear that you'd need to take with you to fly fish Salt Creek.
I couldn't find any fly shops listed in the Salt Creek area; which seemed strange if they have good salmon and steelies(?) But, I think you should take the advice of the other posters, contact someone that knows about fishing Salt Creek, get his/her opinion on the gear that you'll need and proceed to purchase from there. Many times fly shops located a hundred miles or more from where you plan to fish will know what species you'll find there and how best to fish them. It's worth a try.
So, see if you can hook up with someone who knows the area/water. They'll tell you what you need in order to have fun fishing Salt Creek. In general fly fishing gear is not cheap, so it makes sense to get what you'll need if this is where you plan to fish.
Also, there are very few fly fishers that I know that are skillful enough to be able to fish only a single species when there are multiple salmonid species present. Usually, you catch what's hitting your fly; you don't often have the choice to catch a brown trout versus a rainbow trout, etc., since they co-habit the same streams/rivers and have somewhat similar feeding habits.