needing a little help with a bike dillema

Hi,

I have been wanting to do a triathalon for over a year.  I lost about 60-65 lbs a year ago and have kept it off.  I ride almost daily, run several times a week, lift weights and swim a little too.  My treat to myself, to show myself that I am almost in the "fit" range, is to do a triathalon by the end of the summer.  So, I'm planning on doing a tri mid-September.  I have a commuter bike that I ride at least 10 mi a day on.  I run 3-4 times a week, I was up to 5 miles every time, but am back down to three and building back up one mile a week.  I grew up swimming (which ironically is my weakest area), I only swim about two to three times a week for 20 minutes.  I know I need to beef that up. 

The major problem is that my bike is too slow and heavy! I am really tired of it.  I want to get a new bike, but I am a teacher and can't afford to spend too much money.  I found a great website that sells bikes from the factory, I hate not going local, but I can't justify spending what they are asking in the local shops.  My primary use of the bike is as a commuter and for longer rides over the summer when I am off, so the bike doesn't have to be super pimped out, or anything, just something that I can manage one.  If anyone can tell me anything they know about these brands I would really appreciate it.  Also, is it better to have an 18 or 24 speed?  And, forgive me if I'm asking these again.  Oh, and last question before I list the bikes I looked at.  I was at a shop checking my sizing out and it seems like the 50 cm for both men and women's works for me.  I have longer arms, so the women's was a little short, but the sales guy suggested I add on an extender bar, another $50.  I was wondering if all bikes are equally sized.  I see size charts and get a little confused with what I tried and what they list.  My inseam is at least 28" and I'm 5.2, so I have to be a little careful about height.  Anyway, those are just the dilemmas I'm dealing with.  Here are the bikes, if your still reading:

Dawes Sheila (they only have mediums though - 5'6-5'9 clearance) I'm sad because I loved this bike!

Windsor Willow - not sure which size to go with - they have a 44 cm, 48 cm and then it jumps to 51 cm. 

2008 Dawes Lightning 1500 - this bike has clip on aeros, which would be sweet. 

2008 Dawes Lightning 2300 - clip on aeros

2009 Motobecane Mirage PRO - also has clip on aeros

If anyone knows anything about these bikes, or what their thoughts are on what might be best, I would great appreciate it!

Cheers!
Susan