If you've got the bucks, the Cisco NFR Lab Kit Java recommended is a good investment. Though the need for a lab system is highly dependent on what your job role will be and the future of the voice network at your current employer. The lab gear is useful for learning the build process or testing upgrades, but after that it hard to get a real production feel with active gateways and various users groups needing who knows what feature...
Since Cisco has gone the appliance route, going through the steps of building a single site, single cluster has become quite straightforward. Not to make it sound like a cakewalk ... it can get complicated pretty quick. Add on the mountain of add-on application servers and perhaps a global network and ... well you get what I mean.
You can learn a lot just by navigating the consoles, understanding what each and every feature does, then testing those features with a group of phones and perhaps a spare gateway if available. A LOT of what makes up a UCM cluster evolves around the Dial Plan and the associated Partitions and Calling Search Spaces needed to build it; understand it, respect it, and you can do some pretty cool things. Learning in a production environment is a fact of life for many. It "can" be done safely if you're reasonably careful ... make backups, plan and test your changes. And if all else fails and you break something, you'll be learning a lot as you're frantically fixing it.
Good luck.
