The barrel will last longer if you take care of it. Proper break in and allowing time for your barrel to cool between shots (longer for sporter weights, a little less for varmint contours) will help increase your barrel life. 3000 fps wouldn't be pushing that load hard at all so it would increase the barrel life as well.
I can't give you a number personally but from what I read, if you take care of your barrel you could get up to 2000 rounds before its toast. Thats pretty optimistic sounding to me.
Look at it this way. If thats what you want to shoot, do it. If you get something else and don't like it you'll regret it. And in reality by the time you have fired 1500 rounds you've already spent about $1500 on ammo, so whats $500 for another match barrel, or $200 for a factory barrel.
My concern with a 300 is it is easy to develop a flinch with that kind of recoil. Build it heavy and maybe even muzzle break it. Shooting it once is fine, its just once you start shooting matches or just an all day fun shoot, the recoil will get tiresome.
My 2cents. If you really want a magnum, loo at the 7mmRM. The ballistics are better, its faster, slightly less recoil and you can still shoot 168g bullets and even 180g if you have a fast enough twist. Don't let me talk you out of the 300WM if your heart is set on it, because it is a good cal.