Well Believe it or not the .308 will get you to 1000 yards with room to breath with the right loads, even out of a 20 in barrel. Ive had great success at a maximum of 885 yards (max range at local range) shooting a less then desirable 168 gr sierra match king. And when i say great success i mean im bassically making consistant hits on human size targets even in up to 15 mph winds. A hits a hit in my book. And in my opinion the .308 has greater potential accuracy then either of the other calibers you listed.
If this rifle will be used for Tactical style shooting your on the right track with the mill dot scope. Now the next thing to make sure is on the scope is matching turrets (meaning also Milliradians in adjustment) To work with the mildot reticle. Keep in mind the more bells and whistles you have on the scope the less qaulity the overall optic will be. If you do decide to go with an adjustable power scope it needs to be First Focal Plane. If it isnt FFP and is instead whats known as Second Focal Plane then the reticles measures are only correct at a set power (usually max power).
Now the next thing the scope will ideally have is Parallax adjustment. What that does is keep everything lined up at every range for example if your eye is not exactly centered behind the lense and theres any parallax it may mean your POI/POA is now not the same.
So with all that said i will now recommend you the same scope i own and recommend everyone. This scope is tough, Reliable, Repeatable, and has quite good glass in it.
http://swfa.com/SWFA-SS-10x42-Tactical-30mm-Riflescope-P50717.aspxI apaulogize if its a lot of info you already knew but if nothing else maybe another reader can use the info.
Heres a good example of what it costs to add Adjustable power to a scope and keep the same overall quality. It also has side focus and locking turrets...
http://swfa.com/SWFA-SS-5-20x50-Tactical-30mm-Riflescope-P51653.aspxBells and whistles cost money.