If your work pays off the cost of having 3458A (service plan + calibrations) then best way is to go right to the Keysight.
Because any other options are not giving you any safety if your works depends on it. Let's say you even found and bought never used 3458A for 4000$, specified by seller as so, and passing all tests and stable. Ok, great deal, right?
Then after 3 months of happy life ADC hybrid goes sick, and you are looking at instant 3900$ repair cost (new ADC board + calibration), with few weeks of downtime. So you pay 7900$, which still leaves you w/o warranty
Not such a sweet deal anymore, aye?
I'd separate hobby use and pro use and make decisions accordingly
Having myself 3458 I am ok to bear risks to have box w/o warranty, as I do calibration myself, and all non important stuff like bad calrom chips, old caps, busted panels are not worth the time to spend to ask seller. I can see enough on photos of the unit to commit or not. In worst case I'll have dead unit, which is bad, but that only means that some of hobby projects at home will need to wait, not the lost business revenues. My usual threshold for buying parts/broken units is usually no more than 20% of market price for good boxes. That means below 1k$ for 3458A, which are not very often deals on places like eBay.