As I understand, that plan is to cosmetically restore #6 for display in the newly restored engine house.
I think it makes sense to stabilize the locomotive with first class rebuilding and repair to the cab, general structure. Even if it were not operational, the sight and feel of #6 once again slumbering in the Q&TL engine house will be something to see. Even that type of restoration will be daunting if done authentically with all the details.
There is only so much that can be done in in a given amount of time, so it might make sense to bring as much of the Q&TL artifact up to the basic presentational level first. There is quite a bit of derelict rolling stock on the site that could use some organization and possibly culminating in a few new pieces made from all the parts of the car ruins.