I spent ages looking at the options and decided the manfrotto suction pads were worth the extra money. Nice and strong and don't damage the cars (very important).
You'll need some superclamps, I use 4 connected with a connecting rod to give me total flexability. I've an extending rod that's not too long but is really strong (I've hung a broadcast camera from it), but for longer distance shots I'll be manufacturing something using aerial poles (as George Williams uses) and some cable tensioners for rigidity, strength but lightness.
I've more or less ended up with the SM11 from Hague.
http://www.cameragrip.co.uk/acatalog/hague_sm11_car_suction_mount.html
The poles on the suction pads mean you got some angle adjustment on the end of the pole.
At the end of the pole I use a universal joint and mounting plate, such as this:
http://www.cameragrip.co.uk/acatalog/hague_sc2_clamp.html as I bought this to mount on the grabrails on my motorbike.
You could use a magic arm for extra flexability, but they are around £100
On top of that you'll need a wide angle lens, a remote trigger and I'd recommend a decent ND filter so you can easily lengthen the exposure time.
I'd also recommend a bottle of water, a chamois leather and a microfibre cloth or two to ensure the surface you attach the pads to are clean
I bought a lighting bag from ebay that fits everything in easily, like this one:
http://cgi.ebay.co.uk/Kit-Bag-Light...graphy_StudioEquipment_RL&hash=item1c11a85215
Cheers