Before you spend all that money, see how far you can get with this:
http://www.usbwifi.orconhosting.net.nz/

That website annoys me because all of the useful information is held in the images themselves. This one talks about a 10km link experiment:
http://www.usbwifi.orconhosting.net.nz/alfaaudit.jpg

It's not 20 miles, but it's cheap to experiment. I built one, with a USB wifi dongle, a 5m USB extension cable, a garden hose joiner and a Chinese hot oil mesh skimmer. I could pick up APs over 9km away without trying too hard. Of course, if you have a parabolic reflector on the other end too then range will increase (but it's tricky to line up).

Oh, and to answer your orginal question, Hams cannot easily set up internet infrastructure at the drop of a hat. The minimum service you should expect is to be able to relay voice messages out to somewhere. Anything more than that is highly dependent on operator skill, equipment availability, terrain and location, factors which are highly variable.

HTH,

A