Are 500 hours too many for a used hi-end cartridge?

There is no direct correlation between price and quality when it comes to $1000 or $2000 cartridges. More important is perfect tonearm/cartridge matching, capability of your phono preamp if you're using LOMC and your personal satisfaction with one or another model, they can be low compliance or mid compliance, silver soil or copper soil, different stylus profile and overall different sound presentation (different flavor).  

Before you will buy any LOMC ask your dealer about service, because when the stylus will be worn you will have to replace it with equal diamond, you'd be better if the manufacturer can replace old cartridge with new one for special price (many manufacturers can do that), they have special program for upgrade, customer support via their official dealers only. 

Art-9 from your wish list is probably best bang for the buck when you're looking for high resolution NEW low output MC with decent stylus profile. You can also look for Dynavector and Shelter cartridges. 

Current analog front ends are:
Hana EL, Pro-ject RPM-9 with Evolution graphite arm. EAR 834P phono pre.
Vintage HOMC Adcom Crosscoil with Soundsmith ruby upgrade (really), Oracle Delphi V with granite plinth, SME 309 arm, Manley Chinook phono pre.
Both systems feed vacuum tube amps.
Previous favorite cartridges included Ortofon Cadenza Blue, Sumiko Blackbird.

Look at it this way. The vast majority of the sound of a cartridge comes from everything other than the stylus. The vast majority of wear on a cartridge is only the stylus. Therefore the one thing most likely to be worn out is also the one thing least likely to matter. 

As usual when stating something perfectly obviously true people will scream and argue this cannot be. Yet it explains so well the experience of so many who have used and enjoyed many really high end cartridges for years and years and thousands of hours. 

But then I am a genuine example of what someone around here called an iconoclast. I just don't see how hours matter. What are you gonna do? Scope it every hundred hours? And then what? Oh, its worn. Guess I better replace it. Even though it still sounds better than when it was new. I don't think so. What you are gonna do is use it until you feel like getting something better. 

You are never even once gonna look at the stylus. That's what I think.

Maybe ask the seller to take a really good picture of the stylus?

Pertaining to my situation of being a “tight budget audiophile” (if there is such a thing):   If one has an inexpensive turntable with the stock tonearm — in my case a Denon DP-300F with an Ortofon 2M Red cartridge — does it make sense to put a more expensive, higher quality cartridge on that tonearm?

And, given that the records likely to be played on that turntable are 40-60 years old and played on equipment of lesser quality than the Denon . . . like a Penncrest (JC Penney) portable record player with the included ceramic cartridge from 1965 . . does it make sense to play them using a higher quality modern cartridge than the Ortofon 2M Red?