I wouldn't worry too much about the differences in electronics, if it breaks down it won't be hard to fix under the instrument's warranty.
Over the years the guitar's electronics went through some significant changes but as far as I know all new guitars are fitted with the Expression System II
I've only read about problems with the 9 volt battery size, which can differ from manufaturer to manufacturer.
If it doesn't fit, don't try to make it fit but stick to what's adviced in the guitar's manual.
The difference between ES I and ES II isn't exactly clear to me, however a recent post suggests that ES II has some improvements over ES I.
Older Taylors are fitted with L.R. Baggs electronics recognisable by the more visible EQ's. Some people prefer these over the ES but I believe they're more sensitive to feedback.
Alternatively you could buy a guitar without any electronics and fit 3rd party hardware, there's a wide choice in a wide pricerange for those. However I've never been a fan of these as they all are somehow mounted to the top and influence the guitar's vibrations, or may need expert skill to mount them properly. Like drilling holes and wiring in such a way that the wires don't cause additional noises.
Ludwig
