A wise decision!
1. No. They carry the same warranty as new ones.
2. Easy. Stick to RAM from Crucial and http://eshop.macsales.com/search/ram
Switching from Windows to Mac:
1. No, refurb's carry the same warranty as new which can be extended to 3 years with Apple Care.
2. No, RAM is user upgradable.
3. Check out > Apple - Support - Switch 101 Sorry I've had Mac's sense the mid 80s and never owned a windoz computer. However, I do know that an MS-DOS FAT-32 formatted external drive works great for moving file's to and from windoz.
Not knowing your budget situation, with iMacs you really need to consider what you really want to do with this iMac and consider any additional option/upgrades you may need to add AT THE TIME OF PURCHASE!
There are no user serviceable parts inside an iMac. If you just "settle" for a baseline iMac and find out you are not happy with its performance, you need to return within 14 days of the original purchase date.
Then, purchase an iMac you really need.
You may need to consider faster CPU, GPU, larger hard drive or an SSD.
What are you planning to use an iMac for?
Also meant to add that you can't upgrade many components in an iMac. If you attempt an internal upgrade in anyway, this voids the AppleCare warranty.
What you get in an iMac model is what you get. You can't upgrade a particular component, that you may find lacking, later down the road. With the basic models available, if there is something you do not like, you are basically stuck with that iMac unless you return it or resell it to use the money to purchase the iMac you really needed.
Really, I use my pc currently for everyday stuff (web browsing). Pictures, Music. All my media is stored on my network NAS. I am coming from a PC with processor of 3.2 and a total of 8GB memory. I don't do any gaming from the pc. So, the only thing I can think I would want to add later is additional memory, maybe 4gb to 8gb? So, does this void the warranty if I buy the memory and install myself. Lots to think about. Thank you
Adding additonal memory is a THE only user upgrade you can do and NO, it doesn't void AppleCare warranty.
Well, at least, you are not a gamer.
There are quite a few PC gamers that switch to Mac, but an iMac, but don't check the specs of the games that they are playing and what the need to add to an iMac ahead of time.
Most complain about the CPU and GPU not being fast enough and don't do their research before the time of purchase.
You seem like you know what you want and need
Good Luck to you!