You can use any 3G phone that you want to connect to the service, however I wouldn't recommend the Skype phone as its only a 3G phone. If you are using mobile broadband, I'd recommend something that is HSDPA enabled.
The Nokia 6120 Classic is a featured phone which can act as an HSDPA dongle, the only downside is the USB cable can't charge the phone.
3 offer their broadband service on a PAYG basis which might be better for car use. All you need to do is buy a dongle here:
http://threestore.three.co.uk/payg/?modem=1 (which is powered through USB)
Then you pay £10 for 1GB, £15 for 3GB or £25 for 7GB of access (this lasts 30 days).