Welcome to the forum. You've asked some really good questions. Since you're looking at something close to a turnkey minus car, there are other options beside ERA and Factory Five, although both are great replicas and there are many around. Suggest you attend some local car shows and visit with the owners to learn about their experience. Others you may want to consider are Superformance and Backdraft.
I built a Lone Star Classics kit and installed an FE motor in it. Later on I sold that car and purchased a Superformance which is a true turn key minus car. They are built in South Africa, but there is a pretty good network of Dealers throughout the US. Both SPF and Backdraft are complete from the factory minus the drive train. In mine, I installed another FE, this time a 427 stoker, but they will accommodate basically any engine you want. Engine choice is an individual decision and, among other things, how close to replicating an original you desire. All the engine choices you mentioned are work able.
Take your time to investigate the manufacturers and learn their unique differences. Best of luck to you in your adventure.
