It's probably true that in general the later the car, the better it is. But earlier cars will be a bit cheaper.

There have been lots of little and some bigger changes, but overall the car is much the same. If you need more legroom you need the later pedal box but even then the car may require modification to maximise legroom.

Most but not all changes can be retro-added and many of these are not provided without the owner paying. But earlier cars will be cheaper, and all will be low mileage.

I'd go for whatever you like the look of! Mine is an October 2012 car and I've got the later steering rack, but the earlier pedal box (M3W in it). I would like the later rear sprocket and hub assembly for various reasons, but personally prefer the earlier location of the gearlever. The frame is bolted to the chassis better on later cars.

If you can, take a look and try the differences. All are HUGE fun, but be prepared to be an 'interested' owner (which I love), and have a good look round the car with a glass of wine every week or two and enjoy.