Where can I find the total enthalpy and heat of dissolution in water of a given components?
It's very unlikely you actually need the enthalpy.  Like energy (of which it is a variation), absolute enthalpy is almost unmeasureable, normally one deals only in enthalpy changes.  In your case I would hazard a guess that you're  interested in things like dH_rxn, the heat of reaction for the decomposition of H2O2.  You can readily calculate this from the dH0 of the components.  The latter are called the "standard heats of formation" and they are combined with "Hess's Law."  Googling either of those terms should lead you to explanations of how the data you can find can be combined into the data you want. How exactly you do it will depend on what quantity you want, and the thermodynamic conditions (e.g. T and p) under which you want it, and the accuracy you need.

Yes, H is generally a function of pressure and temperature.  The starting search term there would probably be "derivatives of the enthalpy" because you would generally calculate H at one (T,p) from H0 at some other (T0,po). You'll find the derivatives can be related to material properties like the heat capacity, compressibility, and coefficient of thermal expansion.  For changes over significant temperature ranges these can be integrated, provided they are known as functions of temperature, or you are willing to use approximate models (like the ideal gas model) that have formulae.