OMG - I would be very hesitant to buy a bike online without getting fitted. As a woman, most likely a stock bike won't fit, unless it is specifically made for women. Case in point - Terry Bicycles. Plus, paying no more than $400 for a bicycle for a tri...Good Luck. The bike will most likely be heavy with cheap components and break down really easily. In fact, some are almost impossible to keep tuned.
Is there any chance you could borrow a bike from a friend, or possibly buy a bike from a friend?
Another solution is to go to a larger town or city that has a nice bike shop and get fitted. There are measurements that they take. Get a copy so that you know what your measurements are.
I have both stock bikes and a custom bike. On the stock bikes, I had to get shorter stems and on one bike the handlebars had to be cut because they were too wide (should be as wide as your shoulders). You will definitely want a woman's saddle, and I highly recommend the Terry brand - Terry - Women's Cycling Clothing, Women's Bikes, Saddles, Bicycling Apparel, Jerseys, Shorts & Skorts A good pair of shorts wouldn't hurt either - I like the Terry Bella model, and yes, they're expensive too. They do go on sale and that's when I buy.
I'm 5'8" 144#, and have a 34" inseam. I really wish I could tell you about which bike you should buy, but for $400 I really can't recommend any. The only way I'd recommend buying a $400 bike is if it were on sale - say about 1/2 price.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, Guinness in one hand, steak in the other, yell 'Holy Sh**, What a Ride!" - You bet, I stole it!
Current Weight: 151.2
Body Fat %: 32.7