What will be the sign of work done?
Although the IUPAC convention is for work done by a system to be negative for expansion, many physicists and engineers use the opposite convention. What's important is that you use the same convention throughout your calculation. You wrote that H = U + W, which is true only if work done by the system is regarded as positive. To be consistent with the IUPAC convention that you used later in the calculation, this should be instead H=U - W.

On a side note, since work represents a change in energy, not a state function, this equation should be written in terms of changes: $\Delta H = \Delta U - w$. Also note that this equation is only valid for the condition of constant pressure.