If you are not using an amp then Wattage means nothing. You need to ideally look for something with a Sensitivity rating in the 90's.
You aren't feeding the comps much power so you need the speakers to be very efficient at low power.
What ever you do don't buy from CAC. They are a bunch of cow boys. Caraudiodirect.co.uk and caradiosecurity.com are respected online retailers.