The other option, which is the one I tend to prefer, is to set up MODx in a subdomain of your current site. So say your current site is
http://mycurrentsite.com then you can just create a subdomain off of that, say
http://dev.mycurrentdomain.com. Install MODx in that subdomain, hide it from the search bots using a meta robots tag so it doesn’t get indexed, and work on your new build there there. This way you’re building within the same environment where your site will finally sit, and you won’t need to shift databases. Then when your MODx site is ready, all you have to do is shift the files over from your dev folder to the site root, change paths, edit your htacccess file if needed, and you should be ready to roll. Some people recommend running the MODx installer again in upgrade mode once you move your site as this will set all the paths correctly.
The biggest thing, whatever route you take, is to make sure you have a nice solid backup of your current static site and one of your MODx site and database, just in case of anything.
Hope this helps