Buying the right bike for a newbie!?!?!?

In my not so humble opinion, there are three main factors that come into play when purchasing a bike; fit, price, and spec.

If the bike does not fit, it does not matter how well spec-ed or how well priced it is, you won't be happy with it in the long run.

Spec (build kit and frame construction) and price are a balancing act, each manufacturer tries to throw in a little bling of some sort that sets their bike apart from the others at the very competitive price points. Most manufacturers come very close to each other's build specs at the same price point.

 Once you have fit numbers in mind, looking at bikes in your price range from various manufacturers will allow you to compare apples-to-apples as far as the details of each model are concerned.

Fit is a big deal.

Another factor to consider is where you buy your bike. A good Local Bike Shop (LBS) can be a valuable asset, particularly for a beginner. While bike can be had for cheaper from internet sources etc. , a good relationship with your local brick-and-mortar can pay off if things get sideways on you.

Hey Haz,

I'm the same size as you (but I'll skip the heels).  I can't recall my bike size, but will check when I get home for a reference point... but as mentioned above, it would only be a reference.  I have a long torso and shorter legs. 

And one size isn't created equal on all bikes depending on their geometry, etc.  Find a good local bike store (LBS) where you can get a fitting.  They'll really help match the right bike to your frame and fit you accordingly (switching out seats, cranks, stem, even how your cleats are mounted on your shoes, etc.)  My wife had her eyes set on one bike, but it just wouldn't fit no matter what they changed out.  She ended up in something completely different, but she loves it.

And you can get along just fine on your mountain bike for starters... training is training.