I like to order a little extra to make sure I get a number of different shapes and sizes. For a minimalist scape I'd say 20-30 lbs. should be good. I would probably go 30 just to make sure I wasn't short of my goal.
Also, while not necessary, if you have some fellow reefers in your club or nearby you may want to see if someone or a group will share in the cost of shipping on a group order. This would also allow everyone to share in buying a few extra pounds so everyone is happy.
Richard is great with communication and customer service so if you mock up a little drawing with dimensions he'll be sure to get you what you want. While each piece of Eco rock is unique they do come in several similar shapes. Off the top of my head there are medium to large roundish rocks, small to medium holey rocks, branch type rocks for caves and shelf rocks.