Aye, there's the rub.

Here, we have a couple of T1's terminating in the county courthouse, which is up 500' in elevation above our valley floor. With the town up high, and having infrastructure on which to mount antennas for the backhaul (like water tanks, etc), that's how we found our bandwidth anchor point.

The thing to do in setting up a WISP, IMO is to find a place where you have a conventional source of power, the a telco drop coming in, etc, and try to find a location that meets this requirements that would be central to your WISP signal angles.

As for "sufficient bandwidth" -- that depends on how many users you'd have, how much bandwidth they all want, whether you want to pay more for quality of service/compression/multiplexing at either end of your link. 

How far outside Burns are you? From the looks of the Google Earth satellite views, you don't have a lot of high points overlooking the area, and you'd probably need some towers or mounts up on some buildings.