You've got lots of questions to answer before you settle on anything. But for the sake of this response, lets assume you want to be able to kill deer at 1500+ yards. This means you will want a big magnum. I would put the 300RUM (remington ultra magnum) at the bottom end of the spectrum, then you get to the 338s. The 338Edge (300RUM necked-up to 338) is popular on this site. Next step up is the 338-408Cheytac (408Cheytac cartridge, necked-down to 338) it will kill deer well past a mile (1800+).
If you want to have an intelligent conversation about appropriate cartridges and the skills and hardware it takes to shoot at 1500+ contact Kevin Cram. He is an excellent gunsmith and a sponsor of this website. You can see his personal 338-408 here:
MCR 338 - 408 Chey-Tac Re-build
The rifle was used to take a deer at around 1600 yards this year.
Another example of his work are my 260...
My 260 by MCR
For more examples of his work, do a search for threads started by 'Kevin Cram' and you will find pics of lots of his work.
I would also encourage you to call him. You can find his contact info here: