Any long action will work for a 300wm. While finding one in a magnum cartridge would be the easiest, a simple switch of the bolt head would allow you to use say an 06 or 270 action.
As far as stocks go all long actions have the same bolt hole pattern and can be adapted pretty easily to fit either the top or bottom bolt releases. Check out the boyds pro varmit stock. It has the tactical look and have been great for me. I'm not sure there are any advantages to magazine fed or a blind mag setup, I would imagine that would come down to personal preference. I have both and they seem to shoot well either way. The 300wm is a good cartridge especially if your hunting with it, and I'm sure as longrange is enquiring there are several other, possibly even better calibers, if your just looking for a target type rifle.
Good luck with the build. Your in the right place to ask questions as this site is loaded with extremely knowledgeable people.