The HP8753ES is a wonderful VNA but they still fetch a lot of money and they can easily be damaged so my advice would be to be very careful.
At work I think we have about six of the 6GHz versions of the 8753ES. I think they date back to around 2002/3. I've nearly always got one of them on my workbench.
However, they are getting quite dated now and proving less and less popular at work because:
They only go to 6GHz
They are big and heavy
The LCD display is very gloomy and not as good as modern LCDs.
They have a floppy drive but no USB
They don't support the latest USB ECAL modules from Agilent.
They are not supported by Agilent anymore so this can cause issues with repairs/servicing
Other more modern models are much easier to use and are more portable with far better I/O
For these reasons I can only see the used prices tumbling over the next few years. Also note that there are several variants and the 8753ET is the basic TR version. The TR versions will plummet in value. There's also a full 3 port version which is quite neat. Some versions have an external s parameter test set and some have the s parameter test set built in. I prefer the version with the built in test set but there are good/bad points about either type.
A few niggles are listed below...
Be aware that some versions have APC7 connectors so you have to spend a lot of extra money on adaptors to N or SMA.
The LCD display is big but it isn't very inspiring to look at because of the limited pixel count and poor contrast ratio. It looks gloomy! The smith chart display looks quite crude and the smith chart marker text is hidden on the top line of the screen in tiny fonts and looks like an afterthought. There are lots of split/multi display options but some aspects of the UI are not intuitive.
If you make your own cal kit or buy an aftermarket one you will need to use the USER CAL KIT menu in the 8753ES. I can only assume that this part of the HP8753ES UI was written by a disgruntled/incompetent employee at Agilent because it is truly awful to navigate and there are 'wrong' ways to select menu options that appear to be 'valid'. I absolutely hate this aspect of the HP8753ES because I have numerous user cal kits here.
I've also noted some issues with tracking accuracy on big spans if you use the narrower RBW options on some of our 8753ES VNAs. The errors are only tiny but it can be annoying to see a tiny but fake 'step' in the response of an attenuator you are trying to characterise.
My advice would be only spend that amount of money if you need this VNA for business use. Otherwise try and buy an older C or D version for a bargain price from someone who doesn't know what they are selling. The C and D versions are still fabulous VNAs but they look VERY dated when switched off because of the CRT and the dated styling. So definitely more chance of getting one at a knockdown price