An R180 conversion is a pretty involved process- one which I have underway for my WRX. It is not just a matter of sticking in the diff and away you go. Firstly, it will need to have the flange of the R180 changed to fit the tailshaft, or have the tailshaft modified. You will need R180 half shafts, and these come in at least 2 lengths, and there can be differences in their ends as to how they fit the diff. The R180 diff therefore will have different outputs as well, so you will need to pick the right one. The R180 shafts will not fit R160 hubs, so you will need R180 hubs. Some R180 hubs have no ABS provision. You will then need calipers from an MY01 WRX onwards, or STI from 01 onwards, and if you go the latter route you will need the front calipers, and new rims to accommodate them. If you go with the WRX rear calipers, you will need a different disc which while readily available is not on most catalogues- you have to know what to specify. You will also need a new handbrake cable. Make sure the hubs have all the handbrake parts because Subaru keep none or few of them in Oz and they cost heaps. As I said, it is involved and it is all too easy to get the wrong parts if you don't know what you need exactly. And anything R180 is expensive compared to R160, and is harder to source
Can't wait to have the funds and parts to fit one to the back of my subi!