I too would go with the 300 VR on either body, since it also brings the VR to the table, something the Tokina lens distinctly lacks. Something very useful since it really helps when shooting propellor driven aircraft at sensible shutter speeds. Sticking with a Canon comparison the Tokina is going to be closer in IQ to the 75-300 USM III, rather than any of the 70-300 lenses, and miles from the 100-400 V1. It's only really been since the advent of the 150-600s that the third party manufacturers have matched the quality of the 100-400 V1 at the long end of lenses that are longer than 300mm. Also not knowing Nikon does the D7100 have screw drive? As the Tokina, and a lot of other really cheap third party lenses in the F mount require it for AF to function. It seems that Nikon are really good at not specifically mentioning it, relying instead on users knowing the model specifications for the various lenses themselves.
I haven't heard of the Photographers Bag, but as well as WEX and LCE having shops, and doing secondhand cameras in Norwich there is a really good small independent camera shop: Philip's Cameras which is secondhand only and covers the full range of film and digital. It's a proper little Aladdin's cave of kit. I think he will ship, but he's not the type to have a website. The shop is open Wednesday to Saturday 09:30 to 16:30 and the shop is at: 22 Wensum St Norwich, it's next door to the Ribs of Beef! (a very good and well known Norwich pub great for lunch) Tel: 01603 625261.
No I would NEVER buy a camera that did not switch on. Well not unless it was a very modern one, and was under £10. Since all it is is a source for spare parts. I'll leave those to the repair people. Also remember that whatever the issue is is likely to be the common one that causes the fault, so even if you but several they are all likely to have the same fault, and thus not even provide you with a source of a working spare part.
Snydremark since Pagman is going to be regularly cropping his images by 60% to 80%, even with a 800mm lens he would be doing extreme crops, every little bit of linear resolution is going to be important to him. For the sort of images he takes the difference between 16 MP and 24 MP could be the difference between being left with 2 MP or 6 MP in the final image.