You won't need a trim panel for your car @Subru, it is already double din sized. If you move to a single din radio then you can just buy a single din pocket to fill the other half. The factory support bracket should be fine.
As for wiring you can either buy two loom adapters Subaru->ISO and ISO->whatever brand of radio you buy, or you can look up a wiring pinout and just crimp/solder the correct wires from the car to the loom supplied with the radio.
The loom adapters are easy and plug and play but you end up stuffing an ugly, bulky ball of wires behind the radio. I prefer to crimp join the wires but both methods are fine.