Several good calibers but bottom line is you will need either a large 30 case shooting the 240 SMK or a large case shooting the 300 SMK
You will need a gun weighing at least 20-25 lbs lbs with either the 30 cal or 338 and a muzzle brake
Magazine is not needed and just extra money for nothing in my opinion. You will end up single feeding anyway. Second any gun with mag is weaker in the action.
Buy a custom action such as the BAT or Python. Python is $950 and comes with trigger guard and 20 MOA tapered rings in either 1" or 30mm. $950 seems like a lot until you find out how much it costs to true a factory action and then find out it is only worth half what you paid and not nearly as smooth or accurate as the BAT or Python. The Python will handle any case you want.
Three things make accurate LR guns: good bullets, good cases and good barrels.
SMKs are good if sorted base to ogive.
Best cases are RWS, followed by Lapua and then Norma.
RWS cases are now available in larger calibers for $1-1.50 per case and 20 cases have worn out a barrel and then some.
Barrels, Broughton, Lilja, Hart etc.
I would build on the 338 Lapua case. Either in 30 cal or 338. Brass is now less than $1 each and will outlast 10 pieces of Rem in the RUM calibers. Second choice would be the 338/404 on the RWS case.
30"-34" barrel in barrel block with Holland or Answer brake.
Go to www.6mmbr.com
and look at the recommended smiths. Look for Mark King in Pa. He is building on of the premier heavy guns now and is an expert LR hunter. He can build a top notch rig that is centered around his heavy gun stock with barrel block built in. He has several 338/404s that he shoots with also.
I have a 338 Lapua Ack Improved that is built on a custom Rampro action, 32" Lilja mounted in bedding block. It does 10 shot groups at 1000 in the 7-9" range and weighs about 26 lbs, but is a beast to shoot without the muzzlebrake.
As for rangefinders, go either with the Leica 1200 or if longer ranges, get a good coincidental like the B&S, Wilde or French copies.