(Sorry it took me so long to reply, I was at LEGOLAND)
Horses may or may not exhibit signs of ulcers, or if they do, people sometimes think it's other problems. You'd need the vet to scope the horse. I'm not sure of the cost, it varies a bit from vet to vet. If they are present then Ulcergard or Gastrogard would need to be given for about 1 month until they scope clear and then would maybe need to be put on a preventative.
In 'the old days' many people didn't know about ulcers or their symptoms, and would attribute a skinny horse as being high strung and fretting off weight or what have you, know we know better.
Sounds like you've got a good idea of what you need to do to feed your horses.