Custom Built Computer Website

Basically, My business sells specialized gaming and work computers. Currently I am only selling with an ebay shop, I know enough about website writing to build a basic shop and sell them on my own website. However, I do offer upgrades and custom builds and this wouldnt work with a standard online shop. I'm sure there are templates for this kind of website but I do not have the time to look. It will need to be like "[login to view URL]". It needs to be able to display standard products and available upgrades for the products which can be ticked and the price will update automatically. Users should also be able to customise the computer completely just like Cyber Power System's or Dell's websites.