I have just spent considerable time customizing some premade adsense sites I got online.

I must say this is much more time consuming than you may at first think. Consider changing 100 header files, font colors, background colors, adsense ID, adding RSS code etc.  Its not a quick and easy solution at all.

 I think the faster way to go is with templates for specific site builder or CMS apps.

Then just add your content using the application.