12" boulders around the trees arent a bad idea. Id never put edging, block or bullets.....roots with almost always heave them and look terrible, rocks can atleast move with the ground and still look decent.
Stay away from red mulch, looks extremly tacky and unnatural IMO. Black seems to starting to get popular but I would still prefer a brown dyed or cypress over it.
I would REALLy worry about the front entrance before under the trees. Youll have much better luck growing plants there and will enhance the appearance MUCH more then just around the trees. Id keep it simple, 4' rings around the trees with mulch inside and spend most of the budget on the entrance, where people will be walking by day in and day out.