Since you've decided on a custom rifle, you might want to consider something other than the .300 WSM for your stated purposes. FWIW - This is a topic where art, science, and egos get really mixed up. There's a lot of personal preference/prejudice that leads to people taking "dogmatic" positions that ignore rational thought. IMHO - If you're target shooting price, recoil, and barrel erosion are negatives for any magnum cartridge. Custom guns are an expensive investment; I would recommend A LOT of research before deciding on a particular COA. In any case, the bolt face on the donor receiver will dictate the most "cost effective" build-up. That and short action vs. long action.
I believe that .308 is still a respectable performer in long range competition. I've watched members of the US Palma team shoot .308 at 1000 yds. and I certainly wouldn't want them shooting at me. For cost-effectiveness, i.e. minimal action work (barrel swaps- something relatively common in bench rest circles) I submit that the family of cartridges built on the .308's case lends itself to (arguably) better (than .308) target rounds, e.g. .260 Rem, 7mm-08, and .243 (IF YOU RELOAD/HAVE ACCESS TO PREMIUM AMMO!). A close relative, the 6.5-.284, is a recent favorite in long-range competitions. Stick to a family of similar cartridges and the biggest expense is the barrel.
I'm not disparaging the WSM (family), in fact my gunsmith is currently building me a 7mm WSM on a Win 70 action for longrange competition (600-1000 yds.). But - I've already got the other bases covered (and the gunsmith is also a good friend). If I get another action for the right price, I'll look into getting it built in .300 WSM or .325 WSM for hunting.
Gene Econ and Longrange 1947 will probably be along shortly to expand the discussion.
Peregrino