Webparts are typically used for Portal development mode and not ASPX development mode. Webparts are collections of controls that publicly expose properties for the end user/content editor to set. So in your case you don't need the webpart, just the user control that is inheriting the CMS namespaces and displaying the data.
I've personally done many Kentico websites with a few pages to sites with thousands of pages and integrate with different payment systems, web services, etc. and have never used ASPX development mode. Simply because it makes it difficult for the client to take advantage of Kentico's abilities when all the simple settings are done in code and a content editor has to know C# or ASP.NET. Once you learn how to create webparts it's really no different than making a user control.