I have a chance to buy a brand new sealed SE 2020 (2nd Gen) iPhone
SE2020 was discontinued in March 2022, so if it really is "new" it has been sitting in a warehouse somewhere for at least 6-7 months. This would not be good for the battery at all.
Appears that Verizon has now extended all contracts to 36 months, so you should plan to use Verizon for quite awhile before you will be eligible to have the phone unlocked so that you can switch over to AT&T.
Best to contact Verizon directly and confirm on all the details and fine print, as we cannot speak for them.