6 months is a pretty distant horizon. Did you account for changes in difficulty and probable negative market corrections?

All transactions are in the block chain and you can view them in block explorer or via the command line. You only need to guarantee that his address, or at least your address, are the actual ones, because anyone can point to a transaction and say, "that was mine." You might need to show the guarantor that your address is your own, perhaps with a miniscule transfer, unless you are both physically present and view your wallet's addresses or something.

Probably not, unless you can find *really* cheap 5800 series or 6870 AMD cards or free power and cooling.

Nope.

Maybe, maybe not. If you want to try and see if it helps, you can edit bitcoin.conf, which is probably right next to your wallet file, and restart the daemon.

0.3.23 or 2.23?

Can you connect to other sites through your proxy? You want to use the SOCKS port with tor, which will be 9050, 9100, or something like that. Do you have connect lines configured? Do you have irc peer discovery enabled? Are you out of disk space?

You're using Windows. Reinstalling will only make things worse.

Once the transaction appears in a block and that block is confirmed, it is a done deal. The wallet is just a keyring. As long as you don't lose the wallet file, you will be fine.

80 PLUS Bronze is an efficiency rating. You can have a POS 80 PLUS power supply that has a noisy 12V rail or won't run multiple cards, or even one card in some cases.

1x is even fine. Mining doesn't need much bandwidth to the card. (You can even use the physically smaller slots with adapters or modifications.)

The only considerations are physical compatibility, GPU-chipset compatibility (5000 and up cards won't run on nv chipsets), sufficient cooling, and a good power supply. If you are only going to run one GPU, you will probably be fine with either computer and there wouldn't be an advantage in either case, unless you are worried about power consumption.