Why not hire a helmet for your first track day? I think most places do this and it is normally on £20 for the day or even less. That way you can try a track day without the big expense that it could turn into.....
If you do get into doing track days then investing in your own helmet will be worth while but if you are choosing a helmet down worry about what it looks like just make sure its comfy and fits you. Try a range of helmets on and go for the best fitting.
As for the work on your car for a track day, if as you say you are a newby to the scene why not pop down to a local track day and have a chat to people there? Remember your car will be pushed harder than it does on the road so expect tyres and brakes to take a bit of a beating. Make sure they are in good working order before you go and monitor them during the day as you will be driving the same car home later.
Apart from that enjoy yourself!