I'm getting my numbers from Elements of Indution Heating Design, Control, and Applications by Zinn & Semantin.
For a 1/2" round bar, you'll want about 30kHz minimum.
Power calculations get complex, but you should be able to heat 6" of your 1/2" round to forging temperature in a little more than four minutes with 3kW. If you've got a good close coil. Maybe less if you wrap it in thermal insulation.
That's probably not what you wanted to hear, but physics is physics. Your rule of thumb here is 400 watt-hours per kilogram, assuming minimal losses to the atmosphere.
A 15kW machine will work quite well, and the commercial ones adjust their frequency automatically.