1) No. Just remember that a new drive has to be initialized, partitioned, and formatted, using Disk Manager, before you will see it as a drive letter.
2) Personal opinion: Hard drives are commodities. I'd buy a Hitachi or Western Digital or Seagate indiscriminately. The only difference that I care about is green (read slow) vs. mainstream vs. made-to-be-fast.
3) Quite right; current hard drives will not saturate an SATA II connection. There may be some advantage in having more overhead available for burst transfers, but nothing that will matter in the long run. Even if the HDD has an SATA III controller. Oops, I just addressed question 4. They only put SATA III controllers on HDDs because it's cheaper than stocking SATA II for the hard drives and SATA III for the SSDs.