Welcome to X-Cart! If you want to match an existing site, you need to edit the main css file (skin1.css), as well as some of the core templates such as head.tpl, home.tpl, rectangle_top.tpl, bottom.tpl, menu.tpl, dialog.tpl, etc. It takes a while to get familar with which templates are which, but if you use webmaster mode, you can easily at least get the hang of which is which. If you are in a rush to get this project completed, it may be worth hiring a professional for the first job, until you get familar with all of the templates and layouts.
As for the second part of your question, sounds like you need the 'configurator' which is an add-on module that can be purchased from X-Cart.
If you are going to tackle the project yourself, spend lots of time reading and searching through the forum, as most questions have been answered numerous times. If you can't find an answer, feel free to post and we'll help you out. Good luck!
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Padraic Ryan
Ryan Design Studio
Professional E-Commerce Development
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