Good morning,
I bought a Jamis Xenith Pro Carbon/Di2 bike about 3 years ago for £850 pre covid which was in an almost unused condition and came from dealer with a short warranty.
So £1,150 for the Trek seems high to me as the market for used high spec bikes seems to have returned to pre covid prices, nearly everybody who wants to take up cycling has done so.
(A road cc review of the Jamis
https://road.cc/content/review/68753-jamis-xenith-pro )
However I bought the bike because I explicitly wanted to try a carbon fibre frame and Di2 and was prepared to accept the consequences.
New 10 speed Di2 hasn't been made for a quite a few years now, there are no dealers sitting on NoS and used
ebay prices are pretty much full retail.
Remember that that "10/11 speed decision" with Di2 is in the gear mech not the shifter.
Also not long after 11 speed was introduced the software was updated so that new 11 speed mechs and 10 speed mechs won't work together, the software shuts down and no gear changes can take place, the idea is to preserve the image of Di2.
So I would think that unless you actually want to try Di2, buying an 8 year old instance and having a bit of bad luck could leave you in a bit of a dead end; Pay a lot for a second hand 10 speed item of ebay or update front and rear mechs, cassette and chain and go 11 speed.
Or go the other expensive and not always easy to do neatly way and convert the failed Di2 part to mechanical.
I would also suggest being mentally cautious with either bike and budgeting for new chain, cassette and chain rings and hoping that you won't need them, because they won't be £10/£20/£20 :-(
Bye
Ian