I think it would be good to fill out the 104 7U case a bit (maybe not physically, just try different configurations here on modulargrid) then see what it's missing and compliment with external gear. Or do you already have a case? If so, what is it lacking?
Like you said, the Oxi is just a sequencer while the Octatrack is also a looper, sampler and effects box...picking one or the other would impact greatly what needs to be in the case. The Deluge is very powerful but also the most expensive.
All three of your options are good devices, but there's also the matter of personally clicking with them and their own idiosyncratic workflows. I initially paired my rack with a Digitakt, but while I don't mind the "elektron workflow", I soon realized that constantly going between that and patching was too much of a mental switch for me. It's all just deeply subjective.
That said, if budget isn't an issue and you can do additional sequencing/modulation in the rack, I'd say go for the Deluge. It has 2 CV and 4 Gate outs and it's a fully featured device.
The other two options are completely different and would require more work/investigation. The Octatrack might be old at this point, but I don't think it sounds bad at all, it has 16 or 24 bit sampling at 44.1khz, which is basically still standard for bost purposes. The main barrier people usually mention with the Octatrack is its arcane workflow, so make sure to watch a bunch of videos of how it works before you make the jump. It also has no CV outs so you'd need to invest in MIDI to CV module if you want to sequence with it. The Oxi is a very powerful sequencer with tons of CV and Gate outs but that's all it does, so if you also want looping, sampling or sample playback in the rack, in that case investigate your options there (such as Rample, Assimil8tor, SquidSalmple, Morphagene, Lubadh, etc.). Again, subjective, etc.