The engine mounts are dependent on the vehicle. So if you have a Holden Commodore with an RB30, you would use it's engine mounts on the RB25DET. As long as you stay with the RB series and rwd, it should connect to your vehicles engine mounts.
As far as the wiring, you'll need the RB25DET's wire harness and ECU to run it.
The R31 Skylines in Japan had the RB30E and the RB20DET and as I remember the mounts were the same but the harness will be different between the RB30 and the RB25. Have you thought about doing the DET head on the RB30? There is a good tutorial on this conversion here; http://users.esc.net.au/~cobra30/twincam.htm