Good choice in action! I've not heard anything but good things about them. If just for plinking steel or punching paper a 284 Win in a long action or 7WSM would be rather awesome, I have a 284 Win and it will do amazing things and out there a long way. I'm by no means knocking the 300WM but I've seen what the 7mm will do and if I recall the 1 mile record was held with the 7WSM and 180gr Berger bullets (don't quote me I'm going off memory and it ain't Google!) my 284 I'm launching 162 A-Max at 3011fps and not pushing them so for ballistics just do the math.
Now if your plan is to take furry critters at over 1K the bigger bullets might be a better choice! there ar so many calibers and bullets these days that a guy can go grey trying to decide. Either way just shout if your after opinions and maybe a few truths but you can hit at long range with anything if ya put enough lead down range and know what it's gonna do when it gets there. My 243 made 1300 yards several years back with 3 of 5 on an 18" plate BUT there was NO wind

and a blind hog can find an acorn every now and again lol. Best of luck on your build and were gonna need pictures for sure! Later,
Kirk