New triple-boot Kali 2.0/Win7/LXLE 14.04.2, how to install Virtualbox in Kali?

Hello all,

I've been using Linux (various distros) since the summer of 2013, so I'm no expert by any means, especially with the CLI (but I'm learning). I am interested in learning Pentesting/ethical hacking, so I built a "new" computer pretty much for this purpose (to have a "virtual sandbox" environment to "play" in). Not that it really matters, but the machine is is an Asus RoG motherboard/AMD Phenom II x4 965/16gb PC12800 RAM/Samsung 850 250gb SSD/Toshiba 5TB SATA HDD/Panasonic Bluray Drive/Asus R9270 GPU. Not new, but not too bad of hardware, and I do plan to use it for other purposes as well. I've successfully installed the following OS's with GRUB2 for triple-boot:
/dev/sda (SSD)
sda1 (60gb partition) - Kali 2.0
sda2 (80gb partition) - Windows 7
sda3 (60gb partition) - LXLE 14.04.2 (the current distro I have been using for the past year or so)
sda4 (32gb partition) - empty

/dev/sdb is the 5tb HDD, currently storage and swap space.

I DO NOT want to break this system, as I spent alot of time to get this triple boot set up and working correctly (trial and error!). Ok, enough backstory, on to the meat and potatoes....

How do I install Virtualbox on the system in Kali 2.0? I need a tutorial or something to guide me, and all I can find are things related to Kali 1.0. I'm nervous to pull the trigger on using one of these tutorials since Kali 2.0 just came out, and one little difference could either break my system or cause some type of installation issue that will require an untimely amount of time to fix. Also, would it be better to install VB on the HDD as opposed to the empty sda4 partition on the SSD? Looking for some help/suggestions before I move forward. I'm trying to learn alot of different things at the same time, and much of it is thru trial and error I'm afraid. Also, any good places to get info on how to customize the desktop/system, specifically things like mouse buttons (my mouse has 5 buttons plus a scrollwheel that rocks side to side and clicks; I need to map it out to use all the buttons), setting numlock on boot, etc.? I will continue to google and search, hopefully I find something helpful. My wife says I'm wasting too much time on my "new toy".....

Any help would be greatly appreciated. thanks in advance!