From what I can tell and have observed with my purchases thus far, any of those books can enhance your 2300AD collection. Robot, High Guard, and the Vehicle handbook give you the tools for building the relevant gear. There may be additional subsystems, components, or tech introduced in the 2300AD line, but they all build off of the frameworks provided by these books. Additional rules such as expansions to space combat, etc. would be relevant, but depending upon the tech being described, everything isn't applicable. Many of the sample ships & vehicles would not be canonical to a 2300AD game.
The primary factor for these considerations is gravitic tech, which doesn't exist in 2300AD. So any grav vehicles would be non-canonical but anything that isn't grav and is of the appropriate tech level can slot into a 2300AD game. Same holds true for Central Supply Catalog, although I believe some of the items skew towards the space opera side of things. Tools for Frontier Living is supposed to fill the role of the Central Supply Catalog for 2300AD, but I expect a lot of it is compatible if one avoids grav tech and follows 2300AD tech level guidance. Higher tech level items from the CSC could even provide prototype designs/mcguffins for a 2300AD game, if desired.
Companion is definitely worthwhile as it provides options and expands subsystems with broad applications. Alternate character generation options, advancement options, etc.