You can start with whatever you damn please to test your hardware. If you spend more than a year recreating code using FDIL and other methods to get the exact timing (which has been publicly discussed constantly), you can certainly use that code in a commercial product. There is no legal situation when you are not using any percentage. There is no reason to "buy" anything... that is ridiculous. There would be no issues in the U.S. and we have the toughest copyright and patent laws in the world.
As Frederic points out, just to move to a complete AGA emulation you have to throw away most everything from the OCS/ECS chipset support anyways. Alice, Lisa, Gayle, etc. are all new and handle the jobs of Denise, Agnus, etc. etc. You really just can't patch on to the existing minimig code and make AGA work. You have to start over.