I have both an intel and a Cisco AP. The Intel one is 12V which is why I chose it. I would not recommend wiring it directly to the car due to negative spikes, varying voltages etc. I used a power regulator from one of our products at work. This gives a clean 12V @ 1A to the AP.

As far as range goes, it works fine from my house through 2 walls and an aluminium door to my garage. I have also parked it around 100m away from my office in the company car park with no problems.

I have to say that the speed for syncing large amounts of data is piss poor. You may want to look at the 56Meg stuff which is now dribbling into the market (still some licence issues in the UK).

Although 802.11b is supposed to be 11Mb/s, with the overhead you get a real life throughput of around 6-7Mb/s. If I were to start again, I would go 802.11a. At 56Mb/s you will surely fill the bandwidth of the 10Mb/s empeg port.