Is it worth buying used bikes of old models?
In bicycles, the technological advance is not as fast as you might think. This is due to the UCI, which blocks a lot of new technologies or is slow to allow them in races. So I think as long as the older bike is in good shape, there won't be much difference. Maybe it's a nine-speed instead of the modern ten or eleven, but that doesn't really make a difference, especially not if you're not going to ride pro.

That said, be careful about carbon frames and parts. They might be damaged from falling over, crashing or similar, without the damage being obvious. Look for (deep) scratches in the paint job, hairline cracks (especially around joints), discoloration or deformation.