Purchase new Prius + non-dealership/factory PHEV install = $2500 federal tax credit?

Which Federal Tax Credit are you referring to? The one for purchasing a new PHEV Vehicle? Probably not. You would be buying a new PZEZ, and once used (meaning titled to you), you would be converting it as a 'used' car to a PHEV, and therefor not likely get that credit.

As for the "Prius' Tax credit, they went away quite a while ago.

 

If the dealership were to make the modification for you, prior to your purchase, you may qualify. You must purchase a 'new' car to qualify. Until they sell it that way, anything else would not qualify as new. In fact, I would suspect that they would require the factory install to qualify for the tax side of it, as they would likely go off the VIN number to see if it was manufactured that way.

I know that for other vehicle, the Feds use the 'official' factory information to make the determination, so overall, I assume that your answer will be no.

My advice: Buy what you would like, and convert what you will, based on what you like, and not the fact that it would or would not qualify you for a tax credit. In any case, the credits hardly offsets your price, which you must fork over up front, and probably should not be the primary reason for a purchase. (I am saying that because if your primary reason is for the Tax credits, which may be to ultimatly save you money, then I would say that a cheaper Prius without the kit, would likely save you more total money in the long run)