1. Yes you can. But there's one small catch. Most Windows HDDs are NTFS formatted. OS X by itself cannot write to an NTFS formatted drive. However, there are a couple software programs out there that give OS X this ability. Not a big deal. The other option is to simply reformat the drives so OS X can read them. This will wipe the data on them though.
2. Apple refurbs look brand new. The only difference is that they are packaged in less fancy brown cardboard boxes. They have the same warranties as new products. If you can find a refurb with the specs you are looking for, then jump on it.
3. Most PC users get frustrated with their PCs when they start to bog down due to all the crap that the Internet deposits on a Windows installation. In many cases, a fresh installation of Windows restores the PC back to its original running condition. However, surprisingly few people seem to want to take the time to do this (it is a pain to find all the correct drivers and reinstall the 8000 security updates). Instead they simply buy new PCs.
Since OS X doesn't really have these issues, OS X users do not see their machine's performance decrease the same way a Windows machine does as it gradually bloats up with spyware and other questionable items over time.
However, Mac users also tend to spend a great deal of time watching what Apple does next. Half the posts on this forum related to upgrading Apple products in some way, not because the upgrade is necessary, but because the users want the latest (most stylish) upgrade. This is in contrast to the Dell user (no offense--I own two) who rarely checks the Dell site to see when the newest line of notebooks comes out, and then orders a new model the same day it is released.
So, yes, SOME Mac users upgrade a lot. But it's by choice, not because of obsoletion or performance issues. For general web browsing, email, etc, there is no reason a Mac purchased today will not still be performing well for you 5 years from now.
