Welcome to the forum, and wow you do have lots of questions
As far as feed goes, its best to go by weight rather than quantity. I keep a small postal scale in the barn to weigh the feed. Ours get 1 lb. A day so I weigh that out then put it in a container mark the container where the feed hits. That way you don't have to weigh every day. Most feed instructions are based on body weight of the horse and in weight equivalents.
You can give your horse some people stuff but it gets expensive that way so its best to get the horse type. I use DMSO occasionally on my horses. I use the horse type not the people type. Check either your local feed supply or some of the online places like Smartpak.com
Your best resource for health issues will always be your local veterinarian.
I hadn't heard about drug addicts taking bute. It has to come from a vet, you can't buy it over the counter so makes me wonder if its the same stuff