Things to research/learn if you're looking for a job now or down the road: Puppet, Chef, Vagrant.
VMware doesn't really need these tools, VMware themselves have most of this functionality inside their own stack, for the most part, generally the only limits on VMware setups are your budget. The free ESXi will get you a long ways though, you can test pretty much anything aside from HA/VMotion with the free product. Most of the limits are disk space and memory (ESXi free won't boot with over 32GB of ram installed).
With Virtualbox, you'll need to provide more of the infrastructure yourself, this is where Vagrant comes in, and possibly Chef to configure the VM roles.
With a nice Puppet/Chef/Vagrant setup, you can maintain a single unconfigured master VM with Linux distro of choice, and some Chef records for how to turn the gold master into a DB node, WS node, AD server, whatever, and let Puppet clone the VM as needed. That's super-excellent, IMO, a real delivery on the original visions of VM uses from years ago. Really the only thing you won't get with a setup like that is the multi-box-spanning options, which probably won't matter at this point in your progression. It's pretty easy to trigger all the different actions and conditions on a single machine when you're testing.
Also, you're probably CPU-heavy, RAM-light, and I/O/disk space light. If you start going further, I can recommend some ways to grow, but ESXi Free's 32GB limit will get in the way before you run out of CPU.
My ESXi instances, I have one small, slow USB key with ESXi itself on it. It stays in ram once booted and won't touch the original storage till shutdown. You'll use up as much ram as you can add, though, and you'll want it to be as fast as possible. This is why I have such a fondness for LGA1366 and LGA2011 machines. Having 6 or 8 slots instead of 4 lets you hit the big memory numbers with less expense, and VMs want gobs of bandwidth when they can get it. Something like an SSD will be mostly wasted. While faster I/O for your VMs is *never* a bad idea, and it'll quickly become the one thing that haunts you and your budget, one user and some testing scripts can live off of one or two spindles of HDD without much fuss.
Behemoth - Plex server
Galactica - FreeNAS
Pegasus - R710 ESX host
fsociety - MBP 15
Rocinante - E5530 testing machine
Saratoga, Ticonderoga, BunkerHill - the Hydra desktop PC-thing.