Well, I'm predjudiced against spending too much on cosmetic mods. However, OTOH, I've spent around 10-grand on performance mods and fully understand the drive to mod.
I'm a Canon guy, with a 5D MkII, but I generally wouldn't suggest to a Nikon guy to make the switch. You've got a reason that makes some sense to me, so this is one of the few cases where I'd be real supportive of a lateral move between these two great brands.
You don't list the planned lenses, but I'd suggest Ls in every case you later want to move up to full frame. The Canon and Nikon FFs cameras really do reveal the slightest weakness in your glass. You seem serious about photography, so matching up with your buddies and sharing lenses and accessories makes sense. (If you do that a lot).
So, here's my conclusion. You're serious about photography and you expect to get tangible benefit from the camera switch. OTOH, your car mods seem less tangible to me. (Do you expect to get laid more or anything "important" like that? Let us know, since that might override any practical benefit of a camera switch.) Hence, I say go the camera route.
Dave