Off the top of my head, I'd say master what you have first, but even I wouldn't listen to myself

IF you are looking for dog shows, a lot of the events seem to be "no flash allowed," so you definitely want to go for the best low-light body & lenses you can get. That, IMO, puts you in the D700 / D3 range right away.

I don't know much about the sigma lens you mention but seeing the f/f-6.3 makes me think it's not a good choice for any of the uses you mention.

When shooting indoor, no-flash events (I don't really do shows all that much, though I have one next week), I go for the fast glass: 85 f1.4, 50 f1.4. 70-200 f2.8 and 70-300 f2.8.