I'm new as well, but I think I can answer some of your questions:
1) - Irreversible transactions = there is no way. But good news, there are escrow services out there (https://clearcoin.appspot.com/
) that do exactly what you're looking for. With an escrow service like that, you agree with someone to buy something from them for BTC, then you pay the escrow service, then the seller sends you the item, then once you've recieved the item you release the BTC from the escrow service, who then delivers them to the item seller.
If you work without an escrow service, you take a chance - Luckily the escrow stuff is dirt cheap (half a percent from the BTC supplier)
2) I don't believe this will effect anything. But if you were to reformat your hdd without backing up your btc elsewhere, that would be bad and dumb and should be avoided.