I can get a Moog Mavis for under £300 and wonder how useful and beneficial it’d be to have it in this basic starter rack
It's useful...but it also has its own case and power, ergo it's probably not a good idea to toss those aside to spend even more to house it in the more expensive Eurorack cab spaces. Plus, if you really want that Moog sound, it might be a better idea in this case to just get a proper lowpass ladder-filter clone of the 904 and a clone of the CP3 mixer. Those modules are where the "mojo" resides, and the rest of what's in the Mavis can be replicated in a more flexible manner by basic Eurorack modules.
TBH, Moog should've followed their usual pattern of releasing their Moogfest workshop synths, and given us the Spectravox to use alongside their other 60 hp devices. But they didn't.