If you are only buying the laptop for capturing the videos in the field and processing them on a desktop computer, then a Celeron should be fine, but you will need a larger hard drive and a separate 3 or 4 TB external drive to store your captures. The typical size of my Jupiter videos is around 3-4 GB each, Saturn usually a little smaller but if you want to capture the moons also, then count on between 10 and 20 GB for each file!
Processing your videos takes a while - my laptop has an 8th generation i5 with 16GB RAM and 1TB HDD, and it still takes me a good amount of time to process my videos, especially if I'm using WinJupos to process them.
Personally, I would not spend less than $1000 on a laptop, and don't buy old technology 2nd hand, IMHO you are only buying yourself problems.
If I was buying a new laptop, here's a couple I would look very closely at.
https://www.harveynorman.com.au/hp-1...sd-laptop.html
https://www.harveynorman.com.au/hp-1...sd-laptop.html
Andrew