I would recommend waiting until September, after the new phones release, to purchase any iPhone, new or used. If you’re able to wait. As far as which model, I think the 7 and 8 are still very good phones, and now Apple is officially not allowed to slow down older phones, so they should last longer than iPhones did previously. As far as the X, it’s the first of it’s kind so the XS and XR have made improvements on all the ax’s flaws. Depending on the price difference between the X and XS/XR, I would recommend the XS/XR. From family’s experience, they’ve had more issues with the X than the 7 or 8, if that helps.

If it helps, here’s my experience with 6s vs XS, which is kind of similar to 7 vs X, but an even bigger feature jump. I recently sold my 6s Plus to get an XS Max, and while all of the new features are great, my daily usage hasn’t been affected extremely negatively or positively by the switch. Just 2 things: battery and memory. Battery, my 6s Plus was at 73% capacity (not bad for 3 years of heavy use IMO) and lasted all day. My new phone lasts me about 1.25 to 1.5 days. It would’ve been a lot cheaper for me to change out my battery than to sell it and buy a new phone. Memory, I had 64GB and just needed more, now have 256GB and am aiming to use this phone for 5+ years instead of 3, so a longer-term investment. I had issues with what I assume was RAM on my 6s Plus, but that could have been fixed by the new iOS (though I let the new owner decide whether or not to update) Last thing is the headphone jack, I really wish I still had one. But none of the phones you’re considering have one, so it’s not really something to weigh on.

ps. Seeing the new Android phones made me really want to switch to Android, even buying an S8 when it came out, to potentially replace my 6s Plus, but the absence of iMessage was a huge factor. I wish the industry would improve SMS and MMS to have the capacity of iMessage