Re: ? on a new build
I have had rifles built both ways. Unless you are going to really keep your budget tight and only have your smith barrel and chamber your donor Rem. 700, the custom action is the way to go. By the time you have all the work done on that M700 receiver, and for all intensive purposes you should, you will NOT be that far off what a custom receiver build would cost.
I believe Shawn Carlock @ Defensive Edge is a bit of a fan of the 7WSM, and, well, his results pretty much speak for themselves.
FWIW, my vote would be custom action and 7WSM.