You mentioned starting with a Sendero, presumably for cost savings. I would advise against it. That was my thinking a couple years ago too. After sending mine back to Remington to get it to shoot within warranty, which they define as 1" for 3 shots, as measured to the closest edge of the holes (not center) and spending hundreds more having it worked on by a very well known gunsmith, I was still left with a marginal long range rifle that maybe shot 7/8" groups, still had a factory trigger, and did not have an extended magazine to allow for properly seated Berger bullets. I finally sold it and had a custom 300 RUM built with all the necessary bells and whistles--and it shoots one hole groups with Berger 230s--for about $2,350. That sounds like a lot compared to the cost of a Sendero at about $1200, but when you start adding up all the extras---Manners stock, Jewel trigger, 4" extended Wyatt mag, cheek comb riser, muzzle brake
, 28" heavy contour competition barrel, etc., you are actually money ahead with the custom build. And when the rifle is done, it will shoot, whereas a customized Sendero may or may not shoot depending on your luck. Mine was built by Lane Precision Rifles of Pocatello, ID, but I'm sure there are plenty of other builders out there who can build a quality long range rifle at an affordable price.