One thing to keep in mind is the $7,500 Tax Credit is definitely available for 2018 purchases (and to the leasing company if you lease). While it is unlikely Congress will eliminate the Credit, Trump has expressed his desire to do so.
Yeah, the same guy who said he will provide the best and cheapest healthcare.
He will say whatever he wants to placate his base but reality is quite different as the past two years have proven.
BTW, GM is already at the full 200K units sold and pulling the subsidy is not going to hurt them (which is what he wants to do)
I wouldn't worry about the credit disappearing any time soon.