I find tri shorts much more comfortable that bike shorts regardless of distance. My first IM I went for a complete change of clothes at each tranisition. It had a long (17hr) cut off and I wanted to be as comfy as possible. For the second I wore the same tri shorts from start to finish but changed tops in T2.
+1 for getting the tri bars as soon as possible but only after you're fairly comfortable with your bike, gear changing, etc. Feels really unstable the first few times you use them but once you get used to them, they're great. You may need to adjust the bike set up slightly after you've put them on though.
If swimming is your weakest discipline get started on that soonest. It's all about technique as FB says, some proper coaching to get the technique right first is easier than doing you're own thing then trying to correct bad habits.