Much is known about these Hadamard matrices, this talk by William Orrick gives a good overview:
The first few moments of the determinant ${\rm det}\,M$$\det M$ are known exactly, for large $n$ the distribution of $|{\rm det}\,M|$$|\det M|$ is conjectured to be log-normal.
For the number of singular matrices, see OEIS. The large-$n$ limit of the probability that a $n\times n$ matrix is singular is conjectured to be $n^2/2^n$.