There are surviving examples of the international style (ie: conical) visored bascinets from as early as 1360, and the predominantly German klapvisor predates that by at least a decade (though there is plenty of evidence of English using them as well).
With a new impression, your first question should always be "How historical do I want this to be?". Your answer will be based on circumstances, like whether you intend it for living history, actual fighting or just renfaire use, and your available budget. But once this difficult choice is made, everything else should fall into place. If you decide that you're OK stretching history a bit, it is my opinion that due to the fact that period art, effigies, rolls and brasses are so unreliable as a means of 100% positive identification of what was and was not at any one particular point in time and geography, that you should still be OK to stretch the limits by about 10 years either direction. But perhaps I misspoje when I said your first concern should be how historical you want to be. After all, you'll get far more satisfaction out of wearing what you want than being "perfectly uninteresting" to yourself.
Scott
