Curve25519 Key Exchange Bit Security
Provided that you use the all 256 bits of SKEY as key then the bit security of this scheme is about $\sqrt2\cdot2^{125}$.

The reason behind the $2^{125}$ as opposed to the security of $2^{128}$ is because, for curve25519, $2^{128}$ is just an approximation. If we use, as it is more commonly done, the number of "operation" for this calculation, then the number of point additions required by Pollard's rho is about $\sqrt{2^{251}}\simeq2^{125}$.

The reason behind the $\sqrt2$ factor instead of $2$ is due to the fact that Pollard's rho allows to compute batch discrete logarithms faster than what would be a two separate application of the algorithm.
For details, see the section "Batch discrete logarithms" in the original curve25519 paper.