What is your intended use? Do you just want one to have and shoot occasionally, or do you intend to shoot ELR? Do you want to compete all over the country or will you be mostly East of the Mississippi? What does your budget look like? Are you willing to spend $20K or would you prefer to be closer to $10K? Do you have extensive experience in reloading or will you rely on factory ammo? Shooting any one of the suggested cartridges is not a cheap venture. Before you pull the trigger, ask yourself the questions I posed and do the research. If going the custom 50 route, I suggest the Adkins reamer. My last suggestion would be to look at what has done best in Ko2M match. The 50 BMG is not inherently accurate, but I was recently impressed with what the Adkins reamer 50 could do. The 416 Barrett and the 416 Hellfire are great cartridges from what I've seen. I'm a fan of the 375 EnABELR, and the 375 CT. The 408 CT doesn't really offer any advantage over the 375.
As to your last question, brass life depends a lot on you, and how you are prepping it. I know people that only use virgin brass because of the work involved in prepping these big cartridges. Lots more questions than answers, I know. But all things that need considered before dropping the hammer and a lot of money.