By way of getting section going here is a car I built a few years ago from a pre-painted ProtSlot kit. I don't claim its a true scratch build as I did little more than assemble the car and fit a front motor Penelope Pitlane chassis and basic running gear. In addition after I had finished the car I discovered that the number is wrong as the car driven by Chuck Daigh was 15 not 5 as the ProtoSlot images show - the lesson is do your research!
I have a few Proto Slot bodies and they're pretty good - certainly the paint finish on the prepainted ones is lovely.
As to what constitutes a "scratch-build", that's a whole can of worms.
I usually think of collecting all the various bits (of different makes) together and building a car from them is "scratch", since that's where you start from! But I also think that it encompasses more than just putting a simple kit together and requires some skill in the execution of the build, such as modifying/adapting or creating some of the parts.
So on that basis, I would call it a scratch-build....