I think I must be one of the worst herbal freaks (think my friends would agree - the horses certainly would!). I work on the basis that as long as you know what you're dealing with (ie get your vets diagnosis), it's far better to treat a long term problem as naturally as possible - who wants their horse full of drugs? They're not necessarily cheaper than drugs and I'd certainly advise speaking to the experts if you're wanting to treat a specific problem - Hilton, Wendalls and Indian Herbs are all very good.
Ideally you would let your horse choose his own herbs, but I've never managed it. If you're looking for something to feed on a regular basis then the best bet is a ready made general mix. Making your own up is cheaper by a long way I've found, but I wouldn't recommend it if you know nothing.
I know I'm biased

but on my last yard my horses were the only ones regularly fed herbs and it certainly helped put a shine on their coats compared with the others. Give it a go