The 7mm Saum & 7 WSM ( and the other WSM's - 270 & 300) will allow you to use factory ammo.
Personally for the ease of it all (factory ammo) & caliber preference I would go with a 27, 7mm or 30 cal to meet your goals. I would lean to the 27 or 7mm over the 30, but the 30 will get there just fine.
Rear and center grip stocks come as a matter of preference.
Most of mine are center-grip.
Rear grips allow you to use regular rifle triggers which include lighter pulls if so desired (possible magazine for a follow up shots) and a slightly faster locktime. Center-grips allow a better balance and when using a riflescope it is easier to get closer to the ocular.
Joel Russo can get you a center-grip & I think a rear-grip in one of his laminated stocks. He can give you just about any type of forend you want. McMillan & H-S makes center grip stocks as well which are very good.
I would go on-line and check out bullet options (weights/types) with the SAUM & WSM cartridges.
Since you are considering spending this much money to build a LR specialty handgun, why not learn to reload while you are waiting for it to be built (even if you choose a factory cartridge)