Maybe you should not get too blinded by the 8.5 digits.
A Keithley DMM7510 may serve your needs very well. They start to come up used at not insanely crazy prices. Still a bit more expensive than an Advantest or ADCMT. But more linear, less noise, a modern user interface, documentation, spare parts avilable (as compared to the Advantest screen).
The DMM7510 also not a metrology multimeter. I think only 3458a, Transmille and the fluke 8.5 digits would classify as that. But it is not that far after 3458a in most measurement areas. Almost as low noise, better temperature stability, quicker warmup, a modern gui, smaller, lots of other features etc. It lack artefact calibration, but you likely will not adjust it yourself anyway.
Or just realize that you are falling into the voltnut abyss and save your self some future problem and buy a 3458a directly.
![Wink ;)](https://www.eevblog.com/forum/Smileys/default/xwink.gif.pagespeed.ic.cldandycH0.png)
If you find a good used one, you could see it as an inflation protected asset.