The most important thing is to get a frame that fits you. Everything else can be worked out. Go find the Slowtwitch Stack and Reach Database and figure out what are good #'s for you. Use that database to create your short list of acceptable bikes.
After a 10yr layoff I bought a darn nice used tri bike last Jan. I was a little worried that the bike might be too small for me, but there was lots of seatpost so that worked out. But then, after months of trying to get the bike to fit better, I finally figured out that the bike that I thought might be a little small, actually was actually too "long" for me because the top tube was too long. That really took me by surprise. The front end of the bike was also a bit low for me. So a nice bike, but just not a good fit for me. So, based on the bike that didn't fit, I figured out some Stack and Reach #'s that ought to work a lot better. Then I used the database to make a short list and bought a used bike
High end bikes these days are getting proprietary front ends. I would avoid that sort of thing at the first go-round, because they are less flexible in being easily adjusted to fit you. Instead, get a conventional front end that has a fork tube and head tube that looks like all the other fork tube and head tubes made for the last, I dunno, 100yrs.
Look at the fork tubes of some bikes and note how some go higher than others, using some spacers to make it work. New bikes, when they're fitted to their first owner, often have their fork tube cut shorter. The result is a lower front end. So two same model bikes might have front ends 50mm different in height because one got it's fork tube cut. My point is that when you're scheming re. how high you want your front end, you have to include the idea of "cut or uncut forktube" and then match that idea of with a pic of the used bike under consideration.
Don't worry about the # of speeds (rear gears) on the bike. A 9spd rear end can easily be changed to 10. Either rear wheel can be changed to 11spd, but it's about $150 less easy.
Books @ Amazon "If only he had used his genius for niceness, instead of Evil." M. Smart