The battery for the XL is rated 78h on gsmarena which is not bad, but rem that you're buying 2nd hand and the batt might not be able to hold the charge as well as a brand new phone.
I've used the 3T (SD821) and the 5T (SD835) and I already can feel the perf difference between the 2, I'm pretty sure the diff between SD821 and 845 can be felt else there's really no point upgrading. Having said that, the SD821 is still plenty fast esp if you're running stock Android. And last thing for your consideration is Android P will probably be the last official OS Google will support for the 1st gen XL so unless you root and flash custom roms, your updates are going to end whereas for the OP6 you will still go to Q and maybe R even.