1: Be swim fit, this will help your bike time. If you are exhausted from the swim, it will add minutes to your bike time.
2: Bike position should help you produce power, be able to sit in an aero position for majority of ride. Cleat position, I recommend having the pedal axle slightly behind the ball of the foot. You take a bit of pressure of the calf muscle and still have a good powerful stable foot position. Saddle fore aft position, as far forward as possible. You will be able to maintain a better aero position and pedal easier and produce power. Saddle height, Not too high not too low. There are many ways to calculate this, but the best way is by eye. A big consideration is how your ankle flexion is naturally, are you heel down, like Chris Froome, or toe down like Geraint Thomas, or somewhere in between, all are fine, but will effect the saddle height. As will how much you slide forward on the saddle. the further forward you slide (if saddle is level or tilted nose down) the lower the effective height.
Crank length: Shorter is good, but too a point. The slower the cadence, the more leverage gain you will get from longer cranks. The shorter the cranks, allow a less bent leg and so a more powerful leg position and ability to turn the cranks over. However the difference in crank length is fairly small, most options available are from 170mm – 175mm, I most often recommend 170mm for Time trialing. You can open the hip angle most with a forward saddle position. Go for a frame with a long top tube and steep seat tube angle.
Other considerations, such as bar angle, arm rest width, crank length, bar height/armrest height, are all things to experiment with and need testing to see if there is aerodynamic and or comfort improvements.
3: Bike training: You need to build leg strength, muscular endurance, threshold power. So lets keep it simple; 3 sessions per week. Start with your big gear strength work, no need for the leg press in the gym if done properly, think Grame Obree style https://www.youtube.com/watch?v=ePdD-5MfmsU . Then threshold intervals, 4mins fast with 4min recovery, a very good warm up and 5x is a great session. Then a longer ride, can be easy, or as your fitness improves you can include some strength work and TT work.
4: Aerodynamics. First be aware that the bike companies are selling improved aerodynamics on frames, wheels etc, they often use improvement numbers based on 50kmph in a wind tunnel. So take those numbers with a pinch of salt, the real world numbers of those products compared to each other are minimal and..
Your body position is the most important factor, then clothing (no flapping material), head position should be low and tucked in. A good tip is to try to feel as though the air is flowing over your back and around the body, not hitting the chest .
I recommend spending money on a bike frame that allows you to get a good forward and aero position, standard components. Spend a little bit on decent wheels, buy good tires and a good quality new chain.
Relating to Swim, bike and run:
I believe the most important skill for endurance sport, in training and in racing is to be able to feel the correct intensity/pace, to listen to your body.
I agree with Tao Geoghan Hart here: https://www.cyclingweekly.com/fitness/training/the-biggest-skill-any-rider-can-have-is-to-feel-something-and-not-be-told-by-a-screen-tao-geoghegan-hart-a-week-in-training-473806
Having fixed numbers to hit has many problems, not least the power meter may not be calibrated and can vary . Your body changes with a large number of variables effecting these numbers day to day. But your body doesn´t lie to you and you need to learn to listen to it and get used to feeling the correct intensity levels. We recommend using RPE(rate of perceived exertion). You will also save yourself a lot of money. I won´t tell anyone not to buy all the gadgets and the fancy equipment. But you defiantly don´t need it to go fast and to improve.
And remember: Repetition is your friend. Variety is for the weak minded.