Answers to this basic questions are hard to give and there isnt a right or wrong...
1 - depends how much "load" you put into arena ( Layers / effects / resolution / outputs / mapping)
2 - also dpends on the quality you need to have... i did start with 1280x720 or 1280x800 and for most use cases i still am happy with it. I also had a setup were 4k was essential for the design and quality.
3 - there are a lot of ppl who did run arena for days or even weeks. Normaly it shouldnt be a problem to run it for a very long time when you treet it right and dont mess around with the system to much while its running... 5-6 hours is normal for a show... and shouldnt be a problem at all
Laptop: XMG P507 // Intel i7-5500 / GTX-1060 / 1tb SSD / 32gb RAM // Lemur / BirdDog Studio NDI
~self employed AV technician / Schu.VT|a|posteo.de~