Need some wireless help

Here is the situation. Small town police department wants to have some wireless access for their cruisers. They want to make a deal with the local McDonlands's to allow them to put their wireless antenna on their building (close to the highest in elevation in town)

The city will pay for a broadband account at the McD's
They want a 5 mile radius range.

First question:
What exactly is this
Seems to me that it is an access point that has the ability to connect to an outdoor antenna. Could I use this in conjunction with a inexpensive broadband router, and an omni directional antenna like this to do the job?

Or is there something better out there?
Is HomeRF worth considering, or is it dead compared to Wi-fi?
The more I read about this stuff the more confusing it gets.

For a five mile omnidirectional connection, you can throw out the word "inexpensive". You would need one hell of an antenna -- not only for the access point at McDonald's, but in each car as well! HERE is a link to a Cisco omnidirectional antenna that claims a range of 4.6 miles, but that's only at 2 Mbps speeds and most likely depends on a very clear line of sight. Not to mention that it's probably not well-suited for mounting on a car and you'd still need a wireless Ethernet bridge in each car, too.

Another concern is that WiFi isn't the most secure technology out there, even with WEP. If you've got a five mile omnidirectional range, that means anybody with NetStumbler, a packet sniffer, and free time could sit within your 78.5 square mile transmission range and crack the network! (If the police officers are doing anything more than surfing the web, I'd recommend they look into something more secure.)

There are several wireless systems designed specifically for public service use. They are all quite expensive, > 1MB, that's one megabuck, $1,000,000.