Read what Zak posted, I have a lot of respect for someone who shoots longe range matches for g&g.
If I had to choose one rifle for varminting, flattest trajectory, good resistance to drift, I would mention a 6mm-06, 6.5-06, or a 280 Rem. There are a lot of long-range shooters who have a lot more experience than I do, but after shooting a lot of varmints at long and shorter range, one of those would be very high on my list if I was building a long-range shooter. In fact, I am building a rifle this winter . . .
As far as scopes go, a good 4-12X or a 5-15X, 6-18 max.