Archaea speaks the truth. You want a bike that fits properly or you'll be miserable. IMHO you don't have to invest much more than $1,000 bucks on your first bike (especially if you find one on ebay). I wouldn't go anywhere below 105's on the componentry (Ultegra is pretty much as cheap as 105's now anyway).
The main thing you need to remember is to initially work on your endurance with long slow distance. You want to train at a heart rate that is 80% or less than your max HR (220 - your age) and start putting in the miles (minimum 2 hour rides). When you get a base fitness down, then you can start doing interval training and max HR training etc..
Careful, it's highly addictive.
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