1-Pretty sure there will be minimal water passage dramas, you may need to tap and grubscrew the oil feed to the VCT. Dont forget to fit oil restrictors.
2-Simple, use rb25 head bolts, the head is located by the dowels
3-The rb26 also uses a water/oil heat exchanger. I would say both the 25 and 26 IAC would be pulse width modulated, so there shouldn't be too many dramas there.
4-Two options here, aftermarket single turbo rb26 manifold, or an adapter plate. I would avoid modding the head. Cheap rb26 staino manifold would be my choice, just brace it and the exhaust. Or a good aftermarket one-your choice
5-TPS isnt too far from the original, IAC will need lengthening, CAS, water temp and injectors are in the same spot. May wish to add wiring for the Air temp sensor. Will need to change injector plugs, and add a resister pack if you want to use the rb26 injectors.
6-You may have to fit rb25 water sensor, as i said not 100% sure about IAC though
7-As said above
8-Water plumbing for the turbo will be very similar betweent the two, a few hoses to nut out, but nothing too complex