It's generally agreed that 16k in attack (attack + PvE attack) is the bare minimum. I can't say precisely when it comes to Stella NM, but you should aim for 17-18k (attack + PvE attack) or higher.
Honestly, Painters can get by with slightly less if you compare them to Sorcs, etc. I remember totaling around 16.5k attack for Stella NM, which was slightly less compared to a Sorc? Especially since Painter's buffs are strong. Be sure to reach around 60k HP in the endgame with Luna and buffs to avoid being too squishy.
Painter relies heavily on Daeva skills like "Band of Fierceness," which accounts for about 55-65% of total damage depending on the enemy.
In Prometun and Makarna, you can manage with just 45k HP, so Painters can pretty much dive into anything. I just ran Prometun with a 44k HP buffed Painter in a trio. It worked with a bit of extra healing effort 
Usually, people upgrade Legendary gear a bit (sometimes higher for wings and weapons) and then focus on Ultimate with manastones, etc. As previously mentioned, you can easily enhance Ancient gear. It's a viable alternative (though dealing with randoms can be annoying due to their unrealistic demands if you have nothing else).
The highest I've seen is a Painter clocking over 19k attack (physical attack + PvE attack). With that, you can solo Stella EM and clear Tower of Challenge
For manastones, you should predominantly socket critical hit and attack. Precision/Accuracy are not needed. Opinions diverge about crit amounts, but attack is usually advisable. 
I've been testing out the new Aion Riftshade server, which offers these improvements and more with its advanced PvE features. Launching Q1 2026, it's shaping up with balanced classes and excellent latency, making it a desirable Aion private server for 2026 especially since it maintains Aion's classic charm without the typical pay-to-win dynamics. Get hyped!
Doch der zweite Boss ist tatsächlich nerviger