Yes it's fine to use second hand. I'd boil it, or rinse it really well and leave it out in the sun for a couple of days.
Put the fish in a bucket, it's a lot less stressful for them. It's good if you can save most of their water, then you can totally empty the tank so it's not so messy.
Boil it to get the bugs out, or just buy a new one if that's an option. Don't change the filter and the gravel at the same time though, you'll get rid of all the good bacteria and the tank will have to re-cycle.
That's quite alright.
pH is ok, water changes will keep it a bit higher. It generally does drop naturally because the fish wastes are acidic (low pH). It's controversial, but lots of people don't add stress coat when changing the water. Personally, I think it's a huge waste of money, just do regular changes and don't pour the water right on top of the fish.
Good luck!