namely its temp readout for the system temp
That's...a little high, but nothing to really be worried about. 35-40 is about the best you're going to get, in my experience. What's the CPU temp?
And you don't need to worry about it frying your hard drive, either-most hard drives that are even close to modern work fine up until around 55C, and then only start having issues beyond 60C.
That said, replacing the stock fans, which are probably ~30CFM, might not be a bad idea. (I can't find specs on the fans in the case, so I'm assuming the worst.)
I use one of these as my CPU fan, even though it's a case fan. Slightly cheaper options are Scythe, Masscool, and Rosewill. The Kingwin ~79CFM fans I have in my case aren't available anymore, apparently, or I'd recommend them, as they were $5 a piece.
And yes, all three spots your case has a fan for are 120mm, presumably 120mm x 120mm x 25mm, so don't get a 120mm x 120mm x 32mm like this one, as it may not quite fit in all of the spots.