So the complete steps for you would be:

1. Collect the TOS and other license info from all the sites that you would wish to use this way.
2. Collect the robots.txt files.
3. Collect all of the information about your company.
4. Contact a lawyer and get a legal opinion on what you are trying to do.


These steps will get you good insight as to where your possible liabilities would be for offering this as a service or product. You will want to actually go through these steps as when you step over the threshold of doing something like this for personal use to doing it for profit you become a much bigger target for legal actions. I personally would not do it unless I had a lawyer tell me that there was very solid ground to stand on and I had some sort of company entity to sheild me from liability.