The frames from 2016-2019 should all be the same, I think, just different paint jobs. Is it an SLR7 or an SLR9? $5500 is a lot of coin on a used bike, either way, so you'll have to ask yourself if you're comfortable spending that and not getting a warranty.
Personally, I'd check my local Trek dealers and see if they have a non-2020 SLR with Ultegra Di2 that they're looking to move (2020 marked the first frame revision since 2016, so they I'd think that they'd be more inclined to aggressively deal on the older ones, 'specially now).