Re: 300 RUM questions
Welcome! You'll get lots of insight and opinions here. Mine is below.
If you can't afford to go custom or don't want to wait for one, the Senderos make great LR rifles. One of my rifles is a Sendero in 300 win mag with a jewel trigger and is bedded in an A3 gap camo stock. A true sub 1/2 MOA rifle with 208-210 grain pills.
Of course, I would try and talk you out of the RUM. You'll get a lot more trigger time out of a 300 WM with enough horse power to take an elk at a grand. Especially if you're 'new' to this game, you'll enjoy learning on a WM much more than a RUM. They're less finicky, have less recoil and barrels/throats hold up much better. The 300 WM is old news and the 300 RUM is younger and sexier but she carries STDs.
On a RUM, you'll learn how to flinch and by the time you work up your loads, work out the kinks, get a little long range time under your belt you'll be re-barreling.
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.