Hey Chad,
As I see it, there are two main approaches you can take...(sorry this is a little hard to address in a forum post, without knowing your existing application/business)
Set up your site, create static content, use 3rd party modules, and then convert your existing application(s) to module(s)
Depending on what your application is and what it does, this could be really simple or really difficult. It may be as simple as turning your pages into controls and creating a module definition so you can drop it on the page, but if you have logic that depends on the current user, there may be a fair amount of work to do ...or if you are using 3rd party componants/javascript libraries/etc there may be some issues...
Set up your site, create static content, use 3rd party modules, and then link to portions of your existing application
If there are only specific pieces of functionality that are used, it may just be easiest not to integrate, but to link together....if your skin and master page is simliar, it may feel as though you're in the same site still.....there tend to be some flow/navigation issues to worry about in this scenario though....
And remember, not everything has to be a DNN site or module. If you want to take advantage of editing your own content, and all of the existing modules out there, that's great. DNN modules are especially useful if you want to use them more than once. If you're creating a DNN module and will only install it once, especially if you don't have an existing DNN site, it may not need to be a DNN module.
Also, there are certainly core and 3rd party map modules out there. Engage Software even has an Engage: Locator module that can use google and yahoo maps: http://www.engagemodules.com/Modules/EngageLocator.aspx
Take care,
Ian