As a quick heads up, Rohan is going to be a harder force to play well. Cavalry in general is very potent on the attack but fragile on the defense. So it takes a lot of practice and understanding of the mechanics to get the feel for them. You can have a lot of fun with them but I wanted you to be forewarned so you don't get frustrated early with the game. That being said, the Rohan Royal Guard / Royal Knights are a solid Cavalry formation with a high Defense (but they are metal models so unless you proxy it will be expensive to have more than one Formation). Their infantry are cheap (both in cost and points) and you can have a lot of them to provide some significant support to your Cav forces. They have some good, affordable Heroes too though some will point to the lack of Epic Strike-based Heroes as a weakness (it really probably is only in power-gaming or tournaments).
The Three Hunters, like most Legendary Formations, is fun and thematic (and can let you have them for a cheaper cost in points) but is actually "weaker" than taking the three as separate Epic Heroes. Again, this shouldn't matter in some games but you may find after a couple games you just want to take them as normal Heroes.
Hope this doesn't sound like I'm trying to dissuade you from your plans. If it's an army you have a personal love for and put in the time to learn to play it well you can have a great time and there are few things as impressive as a few dozen Companies of Rohan lined up on the table edge.
