I think if I were you and had $500 to spend I would look at buying a totally separate computing device to dedicate to Maidsafe farming with the lowest power consumption as possible. This will help you in two ways:
1) You should be looking to reboot your farming hardware as infrequently as possible, as close to never ever is what I am aiming for. The more you reboot your vault the lower your reliability rating will be. I think Windows machines need more reboots than say a Linux based computing device.
2) A low power computing device means you can leave on all the time for less money, the comparison is really the difference between running a 10W Raspberry PI V's a 500W Windows desktop PC.
For the size of the disk I think I would go for as big and fast as possible, you could go SD and I would if I could afford it, I think I remember reading somewhere that the initial optimal vault sizes are not expected to be that big so perhaps a mid size SD disk would be perfect.
Hope the above helps but maybe the below gives an idea on what others are doing, does anyone else want to provide their intended set-up for Jesse?
I have bought the below kit and am waiting to set-up, this will give me 2 x farming rigs with 2 x 4TB vaults which maybe a bit big for an early vault, but don't think I will get arrested for it which is the main thing.
2 x Raspberry PI 2
2 x 4TB External USB2 connected disk
2 x Maplin Phone chargers to power the PI boards (10W draw)
2 x Disk power supply (15 - 20W draw)
2 x network cables to plug direct into my router (I want to avoid wireless)
2 x Mice (will be disconnected during normal running)
2 x Keyboard (will be disconnected during normal running)
If you are not a fan of Linux then don't panic, there are a lot of YouTube short videos that will help you get going, the Rasp Pi 2 is easily set-up and then there will be some final instructions on how to set-up maidsafe, there are some instructions for Pi2 set-up already but think they will need to be updated at the end of this sprint. If you can get yourself a little way into Linux then Im sure I can help via Webex meeting to get you the last mile, just as soon as I've done it myself and am able to help others.
p,s, If you are set on being a farmer on your Windows PC then think you have all the resources you need already and can save yourself $500.