I'm fairly new to Umbraco, and I would like to understand the best way to extend the functionality of the  RelatedLinks datatype.  

• Should I create an entirely new usercontrol using user control wrapper that mimicks the functionality? 
• If so, would the best/easiest way to be to find all the files relating to the relatedlinks datatype in the Umbraco source files and then copy this as a starting point?  (where can I find the source files - I used the web installer, are they on my machine or do I need to download separately?)
• Or is there a way of extending the source datatype without altering any of the core files?

This is a learning project for me, rather than a business critical functionality requirement.  The functionality I would like to add to RelatedLinks is

• Facility when creating the property to set the links type to 'Internal' or 'External' (so the user cannot choose when editing the content as they can now)
• Facility to add a description to the link
• Facility to edit the link