Changing an existing site over is easier in MODx than in most CMS’s; however, like any task involving the use of HTML, PHP, and databases, it does take a fair level of technicial knowledge.
As far as using MODx to handle just the ecommerce part of your site, I don’t recommend this because the one area in which MODx lags other CMS’s is - you guessed it - ecommerce. This is changing with a promising new package that is being developed called VisionCart, but at the moment with Revo, the only option as far as I know is FoxyCart. This seems to be a good package but costs $20 a month. Additionally, properly integrating any kind of addon component into MODx tends to require a reasonable level of technical expertise.
Depending on your exact needs, you might do better to use a more intuitive ecommerce solution such as
Shopify, for example (not recommending that as I have never used it).
I am the last person to recommend against MODx; on other forums I have it in my sig and tend to plump for it constantly

- however, from what you are saying, it sounds to me like you may want to defer your entry into MODx for a more suitable project, and until your technical skills have evolved somewhat.
Hope this help.