For advice on which bike to buy, check out the sticky thread in this forum on this subject.

Seat height: there are many ways to measure this, but perhaps the most straightforward is to take a picture of yourself, while you're pedaling, at the most extended part of your pedal stroke, and measure the angle that your upper and lower leg make with respect to each other. This angle should be 150 degrees, plus or minus 2-3 degres.

Handlebar height: basically, IMO get them as low as you can, and still be comfortable enough during rides of the distance of your race. The lower you can get the bars, the more aerodynamic you'll be

Also, if you plan on doing more triathlons in the future, know that the optimum body composition for triathlons is basically as light as possible (muscle AND fat). At 5'7" I can tell a *huge* difference between trying to race at 165lbs (last season) and racing at 143lbs (this season)--especially during the run portion, where I'm around 30 secs/mile faster just due to the weight loss. If your 240 lbs is mostly muscle mass, then your speed gains may not be as dramatic, but it's still something to consider if you are interested in this sport for the long-term

Good luck!