It sounds like a 250W kit will be fine for what you need it for, If you only have an 8 mile commute and the hills aren't too steep then a 24V kit would probably be worth considering (as opposed to the standard which is 36V). You'll save money on the battery as well as saving weight because of the reduced number of battery cells inside. Most kits weight about 6-8kg all told and this is definitely worth bearing in mind because you'll noticed the added load on the bike.
Cyclotricity kits are very well regarded, as are the kits from OxyDrive, ElectricBikeConversions, Ezee, ElectricBikeSales (and also ours), worth checking each of these out and comparing features. For instance if you just want the kit for assistance on hills you might like to have one where you can throttle up to max power from a throttle on the handlebar just for those short hill-climbs. As d8veh has mentioned, the LED screen is worth getting as a minimum, LCD screens are price but give you the advantage of being able to adjust the top speed (for off road usage) and show you your trip time, distance and a number of other features.