Each of these is a reasonably involved calculation (or bit of research). Many of the parameters (fault ratings for example) will be a result of computer modelling of the system. Impulse and insulation levels are from standards and depend on the insulation coordination you are trying to achieve. CT parameters are calculated dependant on the system, relays and other devices. The IEC and BS transformer standards give guidance on specifying transformers (and for an 80/100 MVA transformer I think it is worth having a good understanding of these).
I think at this stage (for the project) someone should be using software to model the system, understanding the relevant standards, understanding the operating requirements of the transformer (and associated switchgear, tap-change control, etc). This all goes together to form the requirements of the specification. If you company is regularly involved in power system design, the would like also have existing specifications which could be modified for your case.