Turing machine halting in at least 100 steps on all inputs
You have not correctly computed $\overline{L}$. It contains 


machines which halt in less than 100 steps for some input (which is a finite set of possibilities, as you say), and
machines which don't ever halt for some input.


The second part requires a solution to the halting problem, so $\overline{L}$ is not decidable (nor recursively enumerable).