T...I'm unsure about the license issue is that on eBay UK there is a seller offering the program with the words "commercial license" in the description...
+1 on lightroom, -10 on NX2 as it adds an unnecessary step in my workflow. It can be useful at times but LR is 1000000000 times better - IMO.
Commercial license is just the "standard" licence. Student versions can not be upgraded, commercial can. That is the only difference. For as cheap as it is, just go with the Commercial.
I think you got how it works. I just think of it as placing a photo on a mat, then overlaying clear plastic and drawing on that plastic. Lightroom stores the plastic, and overlays (like a layer) onto the photo. Instead of "Save" you export your edited copy with settings for it's end use (internet, FB, here, etc), and store that somewhere else (at least I do). Once you are done with those, you can toss them as everything is saved in LR.
Space...well, yes it can get big. External drives work if you are using a eSata, fire-wire, or thunderbolt connection. Network drive (what I use) is ok but is slow to load. USB 2.0 is really slow.
To give you a reference point - I have an older computer with a 500gb drive. My main photos and Catalog are on that drive and with about 80k of photos, it is 1/2 full and I have tons of software on it as well. My photos that I use for internet postings (exported from LR) total 3,500 files = 4.0 GB. I keep them in a desktop folder (as they are not originals) and can delete them anytime.
I have moved all my music, personal files, movies, and anything else large onto a Network drive (or usb) as those do not need the speed that LR does. Really old photos, and archive photos go on the drive as well. That cleaned my drive off for LR and my Photos. I will need a new computer soon though. Ideally a internal 1tb drive for photo storage and a Solid State Drive (SSD) for the program and catalog would be ideal.