Sorry forgot that you are 12, paint balling then
Looks like you are getting good to go.
I did not see how you were going to talk to your HDD. You will need something like http://www.amazon.co.uk/dp/B00.....B000LRZG88
You will need a 3.5" version if you have a 3.5" HDD.
Also make sure that the enclosure you buy has an external PSU. I am being lazy and have not checked.
Do you have a USB hub? You will need a powered USB hub for about £5.
So I have added about £10 but you can save £20 on the case.
I would ditch the £20 case and convert an old toaster, old laptop or something. All you are looking for is a mechanical case for protection. Literally anything will do if you can get someone to help drill the holes. Usually the metal is so soft you don't even need to drill, punch a nail and then screw in.
Can you imagine an RPI toaster server? You could fit CD or SD drives where the toast goes.
You will just need some mounting pillars which you may get for free from your local PC repair shop. Remove & don't use the mains then you're safe.
Just an idea but it could be so cool (& cheaper).
Or a lego case. Once you have the shape you may want to glue most of the bricks except the lid.