You cannot use webparts in the aspx template development model. Webparts are made specifically for portal templates. What you'd use in aspx would be the actual user controls the webparts are using.
If you're concerned about your client making changes and breaking THEIR site AFTER you've done the work, then so be it. In my opinion, they need some training then and a phone number to call WHEN they break the site. Don't develop the website in such a fashion as to make it so the client can't perform updates, all you'll end up doing is making the client mad and going to another developer because you didn't implement it in such a fashion that they can use the CMS's functionality.
Using ASPX, your limiting the clients functionality within Kentico that they paid for. It would be like you purchasing a car with all the features but the dealer decides he is going to cut back some of the features because, "You have 5 speeding tickets and 3 accidents, so we're going to disable the radio, navigation, govern the speed of the car to 55 MPH and take away your power windows and locks".
(shameless plug for my blog) I wrote a blog post a few weeks back about Kentico best practices and development models was the first thing I talk about. Check it out. Educate yourself before you go down one road over the other. Even the most non-technical client can be taught, think of it as an opportunity to educate the client and earn some more income on a project vs. being "Big Brother"!