Long-range shooting requires a combination of:
* appropriate caliber (bullet and vleocity)
* accurate (consistent) ammunition
* accurate (consistent) rifle + sights (optics or irons)
* homework (for drop and wind values)
* shooter skill, to execute the shot and judge wind
If we can assume rifle and ammo accuracy are sufficient, and the shooter has correct "drop" data for the distances, it comes down to shooter skill, both in executing the shot and judging wind. Wind is really the crux of long-range shooting, at least to 800-1000 yards.
Because of this, I recommend a "decent" rifle and scope combination, and a caliber which is neither expensive nor unpleasant to shoot. This allows the shooter to develop his skills. I believe every LR shooter should have a 308, because it allows a high volume of practice, and it can "make it to" 1000 yards.
After a year of shooting with the 308, re-evaluate what you want to accomplish, in what forums, with LR shooting, and make a decision then about what better LR cartridge to go to next.
There are a lot of good LR cartridges, and the common thread is the ability to shoot high BC bullets (0.600 or higher) at 2850-2900 fps or higher. Some of them include 6XC, 243WIN with heavy bullets, 6.5-284, 260REM, 6-5.08AI, 7RM, 338 Lapua, 408 Cheytac, and 50BMG. The latter three are very expensive to shoot, but do perform well at ultra long range.