the new/used debate is a can of worms in itself, and there's no doubt there's lots of bike to be had for 1500 if you're comfortable with the used route. if you're dead-set on needing ultegra level spec or you're a weight weenie, used may be the route. if not, I think getting one of these great alloy bikes new is the way to go. you can upgrade gradually if you want and with a few key upgrades you won't be giving up more in weight or performance than some 5000+ carbon bikes. and you get the security and perks (warranty, dealer support, etc) that come with a new bike purchase.
caad and allez are great bikes and seems like they've been the two "standards" when it comes to an alloy bike that can rival carbon. trek seems to have come to that party hard now with the emonda ALR. the ALR 5 is 1659 so a hare above the 1500 point but it's got new 105 R7000, and DAYUMM that purple paint job...
https://www.trekbikes.com/...64/?colorCode=purple. for another 100, you could get disc brakes too.
I think these emonda ALRs are going to join the allez/caad for years to come when it comes to just overall value and quality alloy frames.