Well its a start but please look into those options, in terms of what you will get for the money, that's not a cheap price necessarily.
With Modx what you need is the html and css for the theme, along with any pictures. Not every theme will be easy to get going with Modx.
For instance, any menu you need will need to be made in Modx, you couldn't just import one (at least not a dynamic one).
You also may need to utilize bootstrap or another framework, so that the site will be adaptive to the user's device and screensize. That's not very tough but my point is the theme probably won't include that...hmm I might be wrong on this issue.
You might want to look into themes for Modx, its much more likely you will be able to utilize it more directly than a random theme.
Anyway just ask if you have more questions.