"how difficult" actually has two answers. . .as far as figuring out WHERE to set it up, if you're a small business like we are that's tricky. You might consider a virtual machine on VMWARE or VirtualPC or something of that sort. . . But since this'll never be a production server, a machine that is too slow to handle real mail traffic (or web clients) could still work as an IMAP client for one user (you) as long as you have the hdd space.
The actual process of restoring one of these backups isn't all that hard--I have done it after screwing up a couple of times

. As you no doubt know already, the Open Source backup is basically a copy of everything in /opt/zimbra. Basically you:
1) Set up a dummy version of your server using the EXACT SAME version of Zimbra that you were running when you did the backup (that is, you install it using the same binary--good reason to keep your current binary handy).
2) Copy the entire contents of your backed up /opt/zimbra over top of the real /opt/zimbra that you just created by doing the setup.
Yes, it can be pretty big; I have about 30 mailboxes ranging in size from a little bit of nothing all the way to a couple of 375-MB behemoths (don't ask) and my backup file--tarred and gzipped--is about 2.75 GB. But all it takes is time. . .and luck.
Dan