I'd suggest you attend a traditional shoot somewhere, meet some people and shoot numerous bows. It may sound simple on paper, but there are a ton of variation of longbows, from primitave selfbows, English, American, through all the wacked out reflex/deflex short bows that are neither long nor straight. All having different characteristics, handle/grip profiles, feel at the draw and loose, etc. Even when looking at something plain jane simple as a Hill style longbow, you still have numerous variations such as string follow, straight, and backset, along with reverse handles.
So when you are brand spanking new with zero traditional bow experience, and you say I want to get a good used longbow, the answers can be all over the map. Get out there, pick up some basic fundementals, shoot a bunch of bows, and get behind something that you find easiest or easier to shoot. This may not even be any sort of longbow, and you may find some other form of traditional bow much easier for you as a starter bow.
I'd suggest something simple along the lines of a starter target bow that is very forgiving and allows you to develop technique.
