I wouldn't hold my breath on finding a used Challenger for 50% off, at least not an RT. If the plan holds true to keep this at a 4-5 year run then cancel it, I would think they will hold their value pretty well. It certainly will not hold it as well as the Cuda, but should not be too far off. Besides, if you're looking at a $40K Cuda (?), when you add in the 5% interest on the loan, you lose some of the benefit of the car holding it's value.
As always, it's your money so do what you want, but I just do not like the idea of financing an old car, FWIW.
As a side note, I was at Carlisle last weekend and watched several very nice Mopars go through the auction, and none of them bid up over $30K, so at least the bidders there did not see them as holding value like they used to.