The horse troughs are perfectly safe to use provided that they're not made of metal, especially the galvanized ones; the plastic ponds & Rubbermaid troughs that are sold in many stores are great for fish, though.
The cheapest way to start from scratch is still buying the plastic pond or Rubbermaid trough (at least according to the prices on the Tractor Supply website); the 8'x 2' circular plastic stock tank is the same price as the 8' x 2' circular galvanized metal stock tank according to the local prices for my nearest store.
I would agree but I found a galvenized 450g stock tank for much cheaper than I could get a 300g plastic stock tank.