It seems redundant to build a 280 if you already have a 7mm Rem. Mag. If you are looking for a medium range deer cartridge many smaller calibers with lighter than 140 gn. bullets will do the job quite well. I have many friends that routinely kill Whitetail, Axis, Fallow, and Feral Hogs with 204's and 220 Swifts. I don't think this is nescessarily a good thing to do but with correct bullet placement these small rounds will dramaticly kill a deer out to 300 yds. These guys shoot the aforementioned animals at the neck-skull juncture.
The point is that good bullet placement is what kills animals quickly, with little or no meat loss.
I would build a rifle in 6mm, 25 cal, or a 6.5. There are many medium cartridges in these calibers like a 243 Win., 257 Roberts, 25-06, 260 Rem., add infinitum.
These smaller cartridges will be more fun to shoot than a 280. They will kick less, wich will help you practice more. You can spend muzzle brake
money on some reloading equipment and then really have some fun.