Access point and external antenna question

Hi,
I am helping a friend set up a wireless network between a laptop which
is stationed in a van (she's a mobile vet) and a Fujitsu tablet
computer, a nifty, and incredibly expensive, keyboard-less laptop that
she brings with her into her clients houses.

So, she's got the laptop in the van, a Dlink antenna (ant24-800) that
needs to be installed outside of the van somewhere, and the tablet with
a Atheros5001x wireless card.

Question is: how to make it work?

She has (or somebody has) tried linking the two computer via a Dlink
DI-514 router, which didn't happen, and asked me to try something else.

Now, from what I understand, it doesn't really make sense to get a
router, an access point would suffice, since she won't be sharing an
internet connection.
Am I wrong assuming an AP would be a better, simpler solution?

Also, should I stick with a Dlink product, since the antennna is Dlink,
or does it not matter?
I know that the Linksys AP54eG has a different connector than the Dlink
antenna, so I'll ned an adapter. Will this diminish the performance and
range of the set-up in any way? 9And, amateur question: there are
adapters for these kind of set-ups, right?)

Any suggestions or insights are appreciated!

Cheers,
Imke