A M700 should be able to hit 400 yards pretty easy with a .308. If you put a match barrel, good stock (plus bipod if you want one), amd a good scope on it, you should be able to reach out much farther then 400 yards. Especially if you put that kind of money into it.
Boyd and McMillian both make several types of stocks. I would look into them. You can go the tactical route, or get a wood stock with a thumbhole (or just a classic stock). Personally, I like laminate stocks because they are extremely stable, and beautiful. The down side is that they are heavier then other types of stocks, but unless you are going to carry the gun around a lot or shoot long distances while standing, It should not matter.
As for the barrel, look into Kreiger, or McGowan. They make good barrels. I would get a heavy, match barrel.
You can pretty much go any direction with the optics. Weaver and Leupold are just a few of the good scope brands out there. Just get a large (around 40-50mm) variable power scope. As for the power, that is up to you. You could get something like a 4-20x to something that can go up to 45x. They also make some of these higher power scopes fixed (not variable). I would get something with mil dot sights though.
Remember, you usually get what you pay for with scopes. But that does not mean you have to drom a $1000 on one. (Someone with more knowedge of scopes then me would be able to help you better.)
Have fun and happy building.