I live in Westchester County and if you want my number I can check it out for you. That job is to big for me but I know someone who can. Concrete is pretty cheap. You can have a cinder block structure and then mortar the flagstone or whatever onto the cinder block. You can also use the unilock wall bricks and just stack them up which is easy to do. I think the Mesa would work good. I can come look at it anytime and give you a better suggestion. If you are in need of lawn renovation or landscape renovation I can give you a price. I am down in Ossining but it's an easy drive to you. PM if you are interested in more info or if you need landscape or lawn maintenance.
P.S it is nice to see someone local on here.