There is no "correct power" for any particular range, just different applications(semantics I know). I use my fixed 10x on my savage .308 and it works fine but when I'm testing new loads from a bench I feel like it might be easier to get tighter groups with a higher powered scope.
Your field of view(area in view) with a higher powered scope is going to be less than with a scope with lower magnification. So if you had two targets seven feet apart at 100yds you could see them both with the 10x but probably not with the 20x, therefore it would be easier to switch back and forth to each target with out having to acquire it in your field of view.
If I had to do it over again I would get the 16x as I'm not doing any tactical matches or anything where I would be switching back and forth between targets. If you aren't going to be needing fast target acquisition(which most casual shooters don't) I'd say go for the higher power optic. I use a 22x on my DPMS .308 and it makes it a lot easier to "aim small".
What makes the SS such a good buy is that it is quality glass that is made affordable due to its low production cost. Being that its a fixed power objective it makes it a lot easier to manufacture. Another good feature of the SS is that it's a fixed mildot reticle which can be used in conjunction with an analog calculator (
http://www.mildot.com/ ) to estimate range. I'm not sure if you can do this with a variable mildot as the size of the hash marks would be different in size the more you increase the magnification.
I don't think you'll need to buy a base, just get some rings(30mm for the SS) with at least 20 MOA of elevation and you'll be fine. If I was you I'd get the 16x(thats what I wish I had done). The SS is also built to NATO spec. The target knobs are a little gummy on mine but it doesn't bother me(doesn't effect proformance) and I heard they've fixed this with the newer models.
Heres a couple of links that I've learned a whole lot from. Zak Smith has a plethora of knowledge on high power/long range shooting and has a bunch of free info on the net. A lot of the equipment he uses is waaaay out of my price range but there is great info none the less.
http://demigodllc.com/articles.php
http://www.6mmbr.com/index.html
http://www.gunblast.com/SWFA_Super_Sniper.htm (ss review)