I say 338 Lapua if you can have someone reload for you. I don't buy the comment on component availability . Perhaps selection but so what. There are a couple brass choices ( Lapua and Norma ) and as far as bullets you will be shooting 250SMKs or 250Scenars . What more choices do you really want ( 338 Amax would be nice )
As far as cost of components you need to compare apples to apples here. For example, from Huntington Dies
http://www.huntingtons.com/cases_lapua.html , Lapua brand 338 LM cases are $243 and Lapua brand 300 Win mag cases are $192 . If you are worried about $50 to $100 for your brass after you just spent $5000 on a gun and scope then you really shouldn't be playing with a $5000 gun
To compare the cost of Win / Rem brand brass to Lapua / Norma brand is silly
Cost of dies will be about $50 more . Bullets are a bit pricier but still not unreasonable. For sake of argument you will burn about 15 to 20gr more powder so its really not a big deal $$ wise
Its really not that expensive to get into a good reloading kit for 338Lapua
As far as your scope choices ( 8-32 or 12-42 ) I think this is a bit much . After 3 shoots down my TRG , the mirage can start getting pretty bad to a point where you are dialing down your magnification
As Mystic stated, if you are just punching paper at long range and you just want a good rifle ( not going for the tactical look ) I would build ( have someone build ) a 6.5
The novelty wears off shooting a 300 Win Mag or 338 Lapua all day ( compared to a 6.5-284 for example ) , especially without a brake