I work on commercial lighting. The socket that the light actually goes into is called the tombstones. The ballast in every fixture I’ve done has been separate from the tombstone. This is for a few reasons.
I • It’s very easy to break the tombstones, especially if you’re new to putting in t bulbs. If you’re thinking about getting this just know that you’re risking having to replace the ballast as well if you break the tombstone. I recognize the tombstone assembly and the only difference between that and what I work on is that this is melded into the ballast
II • You might need to replace the ballast. Having the ballast screwed into the fixture by itself lets you just unhook the connectors between the ballast the lights and the tombstone and put the new ballast up. Having the thing permanently attached to the tombstones means a lot more work for whoever is going to work on it
TLDR: since it’s a good deal go for it but if the costs compare you’ll save yourself a lot of effort and repair costs if you get the tombstones and ballast separate.