Roving wireless setup for remote card reader

I have been asked to look into getting a card reader set up in a car
at our site. The site is about 2 kms across.

I was told that most of the time it would be offline, but sometimes
would be online - when it got close to a wireless access point and I
should be looking at using 802.11g

Is this right? I seem to remember at my last job that we were looking
at getting a wireless network set up that covered a similar size
location (it was a remote township) and the idea was that laptop users
would just be able to use a PCMCIA card to connect to the WAN from
anywhere. There were going to be a couple of base stations involved,
but the total cost was around $50K and that was a few years ago with
no loss of network connectivity?

I am no guru, but I do want to validate the information I am getting.

I'd appreciate any assitance anyone may be able to offer on range or
set up.

Thanks.