Coming up with a homemade Dr. Pepper clone sounds like a lot of fun. Sure, it's probably not cheaper than buying a bulk box of syrup, but where's the fun in that?
All the above if you have it. in the keg if you want, in a post mix system if you have one. Personally, I'd pour it into cleaned Torani bottles and stick a Torani pump on it and mix by the glass whenever I wanted it, but that's probably because I don't drink it quickly enough.
Just to throw in my 2 cents. But 5 gallons of 5 to 1 syrup would make 35 gallons of finished soda. You can't forget the syrup starts out with 5 gal and you add 30 gal of water to that.