It sounds like you have a good handle on the legalities, but on a more practical level if the sites you are scraping don't want to be scraped, it would be pretty easy for them to block you, obfuscate/change the page structure at any time to make your scraping impractical, etc. Of course, you will be able to play along too by obfuscating your source address and improving your scraping, but it could turn into a time consuming game of walls and ladders.

Of course your startup idea may still be worthwhile, but in the longer term you'll be at the mercy of the content owners (who might even be fine with it, or want to acquire you).