There are some good photos of various forges on this site.
http://zoellerforge.com/index.html I went with a blown air burner system because of the simplicity and ability to use the components on multiple types of forges as I learned what I liked and improved my forge designs.
I built a horizontal forge, similar to the little johnny forge, out of an 11 gal air tank. Horizontal forges can have hot and cold spots if the flame doesn't circulate properly due to burner placement.
The vertical "top hat" style forges are great for an even heat but you have some type of stand support to hold up one end of your billet or tongs. It is also helpful to have a mig welder or the like so you can weld handles onto shorter pieces of steel.