getting a bike online...
 

Okay, there's no place to buy one nearby, and I don't have a car. so basically, i'm stuck with trusting everything i've read, and based on all that i plan on getting a trek 520. So, I'm basically stuck buying it online, which means that I can't try it out for size beforehand. How can i figure out what size of bike I should look for without being able to get near one?

http://www2.trekbikes.com/Dealers/Index.php

I would check the dealer search at www.trekbikes.com and either call or email the nearest dealer to you with your inseam measurement. 520's only come in 5 sizes so it won't be hard for them to tell you what size you need. I wear 34" length pants and my dealer ordered me a 23" 520. Fits me great.

520 buyers can make an LBS work for their money:
Take care that someone does not cut down the steerer tube of the forks too much, this will limit your bar height.
Many leaded tourists specify an MTB (shimano LX) crank on the 520 to get lower gearing.
The wheels of the 520 often need a tweak by a good wheel-builder to be strong enough for loadcarrying. The luggage rack another target for an upgrade.
The aim is to make the 520 more like the Bruce Gorden BLT, which is setup how most loaded tourists prefer.