Cannot download Mavericks - spinny-thing of doom, App Store stops responding!

Question

Hi guys,
It's been a very long time since I had to ask for help from the community - please excuse my username, I made my account when I was 16!
I've been running Snow Leopard 10.6.8 very, very happily since 2010. Love the OS so, so much. But the lack of security updates will become a problem as I'm an IT professional, so I guess I'm gonna have to bite the bullet and upgrade. This is under duress as I don't like any of the iOS features that have crept in since Lion, and I've spent 3 years customising SL to how I like it
However, whenever I go into the App Store and click Free Upgrade on Mavericks, that's as far as it goes. Worse, the App Store ceases responding, but it's still chewing CPU. My machine is compatible - it's an early-2008 MacBook Pro (A1260/MacBook4,1), the last non-unibody, but I get no error messages or popups. Putting the cursor over the App Store window summons the beachball, even though the progress spinner on the App Store window is still spinning. Right-clicking on the Dock icon shows it's Not Responding. Nothing seems to be downloading.
I enabled the Debug menu and both cleared the cookies and reset the application, but neither have helped (explains why it's so laggy, it's a glorified web browser!). This is what things look like after 20 minutes of waiting:
If I can't get it to download on my machine, I have a friend with a MBP who can download it for me. I want to do a fresh install anyway; I have a big USB key on order so there's no rush. I've always done fresh installs with OSes because they are so much cleaner; I am not bothered about the time it will take me to rebuild the Mac, if it means everything works properly first time then it is time well spent.
I am very technically capable and don't mind debugging this, if someone can help point me in the right direction The last logs I have in ~/Library/Logs/App Store/ are from 2011. Browsing through All Messages, I found this:
30/07/2014 7:43:56PM    /Applications/App Store.app/Contents/MacOS/App Store[60931]    MDS Error: unable to create user DBs in /var/folders/XX/XXo5fegZExKN+HipTvCqe++++TI/-Caches-//mds
I don't know if this is related because I have a similar error for Google Contacts Sync.
Thanks in advance!

Answer 1

Back up all data.
This procedure will unlock all your user files (not system files) and reset their ownership, permissions, and access controls to the default. If you've intentionally set special values for those attributes on any of your files, they will be reverted. In that case, either stop here, or be prepared to recreate the settings if necessary. Do so only after verifying that those settings didn't cause the problem. If none of this is meaningful to you, you don't need to worry about it, but you do need to follow the instructions below.
Step 1
If you have more than one user, and the one in question is not an administrator, then go to Step 2.
Triple-click anywhere in the following line on this page to select it:
sudo find ~ $TMPDIR.. -exec chflags nouchg,nouappnd,noschg,nosappnd {} + -exec chown $UID {} + -exec chmod +rw {} + -exec chmod -N {} + -type d -exec chmod +x {} + 2>&-
Copy the selected text to the Clipboard by pressing the key combination command-C.
Launch the built-in Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.
Paste into the Terminal window by pressing command-V. I've tested these instructions only with the Safari web browser. If you use another browser, you may have to press the return key after pasting.
You'll be prompted for your login password, which won't be displayed when you type it. Type carefully and then press return. You may get a one-time warning to be careful. If you don’t have a login password, you’ll need to set one before you can run the command. If you see a message that your username "is not in the sudoers file," then you're not logged in as an administrator.
The command may take several minutes to run, depending on how many files you have. Wait for a new line ending in a dollar sign ($) to appear, then quit Terminal.
Step 2 (optional)
Take this step only if you have trouble with Step 1, if you prefer not to take it, or if it doesn't solve the problem.
Start up in Recovery mode. When the OS X Utilities screen appears, select
          Utilities ▹ Terminal
from the menu bar. A Terminal window will open. In that window, type this:
res
Press the tab key. The partial command you typed will automatically be completed to this:
resetpassword
Press return. A Reset Password window will open. You’re not going to reset a password.
Select your startup volume ("Macintosh HD," unless you gave it a different name) if not already selected.
Select your username from the menu labeled Select the user account if not already selected.
Under Reset Home Directory Permissions and ACLs, click the Reset button.
Select
           ▹ Restart
from the menu bar.