Seeing no one has responded I'll put in my $.02 worth. Nilgai taste real fine, but like many critters, the cows taste better. The older bulls maybe a little on the tough side if real old, but still make a darn fine meal and taste great. A large bull will fill up your freezer, they are larger than you may think.
A license is a real bargin. To hunt exotics in Texas you only need a $48.00 special five day license When I was down there last fall the ranch manger told me many guys paid for a regular Texas non resident license ( big $). Don't, it's not required and a waste of $, unless you're going to hunt native game . You get get one on line even or buy one there at any walmart or sports store.
Do a google search and there are a million ranches that you can check out. Check out the exotic forum. I was down there last fall and had a great time.