I am, by far, not the one to give advice as I am just learning about espalier myself. But here's a good article that explains why to cut it down:
http://www.motherearthnews.com/Organic-Gardening/1993-10-01/Espalier-Trees.aspx

I went through the same thing when I bought my SR Plum. It looked like this when I bought it:
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I didn't have the heart to cut it down to 18", so I just trained it from there. I cut off some branches and got this by the end of the summer:
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I ended up having to cut off the top two tiers due to a big mistake I made (constriction), so it looks much different now. I'm really hoping to get back to this by the end of this summer.

Oh, the other thing I'd caution against when using current branches, is trying to bend them too far. It stretches the bark and can damage the branch (learned that last year too! :) )

Apples are very easy to train, very flexible branches so I wouldn't worry too much about bark damage. Since you have been reading about espalier, you must have read that you lower the branches in steps (don't just force them to a 90 degree angle to the trunk).
About the time factor - espalier can actually bring about fruiting sooner depending on the pattern you are trying to achieve. Training the branches horizontally, pruning the length, provokes the little tree to convert the branching buds into fruiting buds/spurs. And then you want to disbud to keep them from fruiting until you get the branch structure grown in the way you want it!
The point of espalier is using available space as efficiently as possible so a gap of 3 feet from the ground is just a waste - you could have one if not two tiers of branches there.

Take a look at the book: Pruning and Training, by Christopher Brickell and David Joyce.

It explains the process and the logic very well, and with pictures. You can probably find it at your local library.