Welcome to MP3car! Sounds like you know what you want and have clear goals for your system so your already ahead of the game.
Look at the Griffin Power Mate for volume control, Wifi is easily done; if more range is needed, external mount antennas are available. XP, 7 or Linux if your so inclined all work great. You will want a front end for the OS taht is Touch Screen friendly so look at Centrafuse, Road Runner if you want main stream. FM tuners are readily available.
I can't imagine you will miss your HU if you do some research here, build what you need and install it correctly.
Good luck with it, search this site a bunch and ask questions on anything the search doesn't answer. Oh, and read a bunch of worklogs on other peoples projects, theres tons of good ideas there and if you search for Subaru's within that forum, I'm sure there are a few there.