Any standard helmet will do. You're right, the only ones specially designed for track racing are very high-end helmets. Some people use those. Most people use normal helmets.
It doesn't have to be new, but a good rule of thumb with helmets is buy a new one every three or four years. Exposure to sunlight and air can cause the material to break down to the point where it may not protect you in a crash.
There are no faux-pas to worry about.
You don't need a helmet with a visor, or with an integrated light.