You may get differeing opinions on this but here's my take:

1. The frame is SL Aluminum and is the same frame for the 1000, 1200 & 1500. It's a good solid frame, but not what you'd consider a "racing frame." Upgrading the wheels and tires will make the biggest difference on the bike performance wise. The Front Derailers that come stock on the 1000 are also kinda iffy. Consider an upgrade to 105 FD. The bike will shift better. The 1000 is also available only as a triple if my memory is correct.

2. If you do not own a modern road bike at all, and are on a budget....buying a trek 1000 and putting a decent set of aero bars on it will be fine for your rookie season in Tri. AS you get more into Tris you'll almost ceertainly want a better bike, but a women I know locally rides a low end specialized with aero bars and does quite well in local tri events. She trains hard and is in great shape.

The difference between Tiagra and 105 is noticeable, and the durability is another issue to consider. Belwo Tiagra is Sora, below Sora is a level of parts with just long number designations and no names. The no name shimano line is what type of Front Deraillier comes on the 1000s. The Tiagra and 105 Shifters shift in the same manner as the Ultgeraas and Dura Ace, the Soras that come on the 1000 use a thumb lever which some feel is not that ergonomic. The Shimano line goes like this:

Generic Shimano (just part number series)
Sora
Tiagra
105
Ultegra
Dura Ace

Hope this helps.