ECP is great
Clear and concise to find everything once you have filtered down by your car. I like service packages, brake packages etc basically everything you need to do general maintenance. This all would take extra work considering you would need to cover all the big makes and models. Might be nice to offer OEM packages v 3rd party packages depending on your margins. Plus try to take the car reg / annual mileage / service date at check out. You can then do some email marketing before the service is due to sell them parts the next time round.
From a business point of view ECP have a slick operation and very aggressive pricing. It's almost worth asking them for a white label!