Yeah, you could buy a basic one and add the rear seats and roof later.
But, is that likely? Most people buy cars on credit and won't have $10k cash to spend on those bits later. I guess they could put it on Visa, but that's a terrible financial choice.
If these become popular it should be reasonable to expect the aftermarket to provide less expensive options. These are clearly where Slate's margin will live right now.
It's possible they will offer financing for that. From a business strategy perspective it would be almost unimaginable that they wouldn't.