Welcome to the forum, OK so I.m new to all this myself so any advice will be limited but there are a few things I may be able to help with. Firstly the ebay supplier you mentioned has been recommended by a lot of the members on here, I am wary myself but from what I have read he is one of the better suppliers & well priced as well.
I wouldn't start to order anything yet though as you need to get the design sorted first. Don't worry about missing a certain item on the dreaded auction site as apparently this guy will put together a package to suit your needs at a good price.
First thing I did was to have a read through some of the build logs, you will find you will pick up a lot of infomation & get some good ideas about design.
Best of luck with it, your in the right place if you are happy to learn.