I'm not familiar with NSW regulations on engineering relating to engine size (not kW) - in Vic it's no more than 10% more than stock without an engineering certificate ($$), but I do know your power-to-weight ratio as a P-plater can't exceed 130kW per tonne, which for a Brumby is pretty much that figure. So in terms of engines you can probably install a N/A EJ25, certainly an EJ20 or EJ22. In terms of wiring, given what you want, I'd highly recommend having a chat to Gannon in northern NSW. I went for a relatively simple swap (no ABS or cruise control), and Gannon's work made it a lot simpler.
I would also speak to an engineer about the ABS in particular, as there could be all kinds of requirements. Putting an ABS system designed for a 1400 kg Liberty into a 950 kg Brumby isn't likely to be as simple as just dropping it in. Unless you really want it, I'd skip it.
Your braking should absolutely be upgraded. The front struts can come from your donor vehicle (with some mix-and-matching of CV's and the top hat), your rears can use the donor vehicle backing plates (need to be modified) and calipers/rotors but the hubs will need to be XT6 (as Bennie mentioned, I should still have a set available from the batch coming in). Your handbrake will also change from front to rear. You might get away with a 4-stud rear disc upgrade depending on the engineer and your engine choice - an EJ20 is probably borderline, an EJ22 or higher I would hope an engineer would require the 5-studs.
I would recommend the EJ gearbox, most EA's are getting long in the tooth and if you want awd you need a Vortex or RX awd box (very tough to find). Your existing box is likely the 4-speed, which is barely adequate for an EA engine. The later 5 speed from a L-series is much better. I believe the earlier Impreza's used a 3.7 ratio, which can match with your diff (it will probably be either a 3.7 or 3.9, but both are easy enough to find). Later EJ's used a 4.111 I recall.
I have seen complete Impreza dashes swapped across into Brumby's. More work in terms of fitting, but it's an option.