For the first two...
>1. What dimensions should I check to be sure the bike fits correctly?
>2. What should I know about components (i.e. crank length, brand
You should probably go shopping. Go to a couple of reputable bike shops, let
them know you're doing some comparison shopping, and ask questions! They'll
help you find a good fit on a bike, and they can tell you about the various
components. Short of that, read some of the bicycling magazines, especially if
they've got a "buyers guide" issue (although usually those are published early
in the spring).
If you do go to a bike shop, do keep an open mind toward actually buying one
there. First of all, the chances of you finding *exactly* what you want on the
used market -- with correct sizing and components -- isn't great for road
bikes, since those are the least popular bikes on the general market. Also, if
you're getting serious about biking/tris, it's good to develop a relationship
with a bike shop. Many will give you free or reduced-price tune-ups on bikes
that you buy there, which is significant, because your bike will take a
pounding in training and races. Also, if the guys at the shop get to know you,
they might give you better deals when you buy accessories. I bought a great
bike at a little shop, and whenever I go back there, I routinely get 10%-20%
discounts on stuff...
>3. Is steel a good economical alternative to more expensive materials?
Steel's pretty much the ONLY economical alternative. Although there's some
overlap, generally the lower-priced aluminum bikes cost as much as the
higher-priced steel bikes.
If you are trying to buy used, do some research into the various properties of
metal vs. aluminum vs. carbon fiber, etc. You don't want to buy a metal bike
that's old enough to be in danger of cracking, rusting, whatever.
If this is a bike you want to be using for several years in competition, think
of this as an investment, and don't necessarily go bargain-ba***t. Unless
you plan on constantly upgrading, a $700 bike that lasts you 10 years is a
better deal than a $300 bike that lasts 2 or 3 years.
Just my two cents... have fun!
Los Angeles, CA