You can try to buy the HP 8510C + 8516A + 83621A, but it is not the latest equipment and you need a lot of space in the workshop...
I saw them and was thinking about it but only broken ones are cheap. The ones that power on and show something on display are almost the same as a not tested but powering on Rohde & Schwarz ZVM. Also the shipping costs will be probably high for that if it is not in a normal driving distance to the seller for pick up.
I'm not in a hurry. It is just for my amateur radio hobby so I can wait for a good deal.
Unfortunately, I also have such a problem and to be honest I have been looking for a decent VNA for a home workshop for many years. Unfortunately, when something interesting appears on the market, it is quickly bought by people who professionally trade equipment, and then sell it at very high prices
Unfortunately, to buy something at a good price, you need a lot of free time to follow the auction (unfortunately I do not have this time), when something appears at a good price, unfortunately it disappears immediately.
24k euro for 8510C IMHO is a crazy price. If I had a larger workshop, I would prefer to buy a few cheap damaged ones and assemble them into one functional measuring instrument. Even with a lot of bad luck, it would be cheaper anyway.
There is even an attempt to use the HP 8515A module on the Polish forum and do the rest yourself. But I don't know how this attempt ended (Unfortunately, the topic is in Polish:
http://mikrofale.iq24.pl/default.asp?grupa=160341&temat=423430 )
Maybe it would be a good idea to look for hams who have such measuring equipment at home and talk to them about the chances of repairing such a device.
I am slowly starting to think about the Siglent SNA5000. I'm still waiting for independent reviews and most likely, if the reviews are good, I will buy up to 8.5GHz. Yes, I would like something more, but you have to think realistically ... and at the moment the most I miss the possibility of measuring in the 5.7GHz band.