Audio upgrade for a prefacelift S

I'm in the process of fitting my LPG kit but I was thinking I should spruce up the interior too while I'm at it and the number one thing would be an MP3 SD/MMC/USB headunit audio upgrade. Has anyone done this on a prefacelift car and what would they recommend? I'm not looking to spend a fortune creating a show stopping setup just something I can play my many tunes on and will sound at least as good as the OEM setup. I'll want to keep the steering wheel controls too and I'd like touchscreen too but thats just a whim.

And I don't want to spend more than a couple hundred tops.