Question 1: I read people advising to only try and access backup files of Wallet.dat?, why?. Could it suddenly mess it up?. And can you simply copy by Copy/Paste?
While it isn't likely, there is always a possibility that something may occur that might corrupt your wallet (bad shutdown, wallet file format upgrade, adding encryption by setting password etc). So, when dealing with older wallets, it is generally advisable to avoid using the original copy too much, just in case something goes wrong and the wallet gets corrupted. By working with copies, if anything goes wrong, you still have the original
And yes, you can simply use Copy/Paste to create copies
Question 2: Is this plan ok??.. I plan to let Bitcoin Core download the whole blockchain after I get new harddrive (150~gig??? insane), and then I will see if I got any Bitcoins, right now it shows my balance as zero?, but Bitcoin Core says not to trust it until it has updated all.
As ncsupanda said, you SHOULD be able to see all the addresses in your wallet file... you could check all of those addresses using a block explorer without having to wait for Bitcoin Core to fully sync up.