The real question you should be asking yourself is if you really want to get into the wireless ISP game. It is pretty late in the game at this point with CLEAR rolling out WiMax service(yes I have heard the negative reviews), docsis 3 and fiber to the home solutions becoming more prevalent. It is a rough time to be getting started. That being said I agree with some of the previous posters that your core equipment should really be Cisco. Think used 6500s and 7200s. I know that a 6500 with sup32s can handle multiple hundreds of mbps of traffic for thousands of users, but it depends on your usage scenario. In this case I would keep the Mikrotiks at the edge as it is quite the mature platform(not that I haven't spent countless nights trying to figure out why it behaves the way it does). If you absolutely must run open source for your core, investigate Vyatta on high end hardware. It is not as mature, but the community is very good and helpful. You might also consider the zebra project for just straight routing.

This is the only thing that I have seen that is close. http://www.theuniversal.net/