Stang70Fastback

Hey guys. I need a little help. I've got a 98 Subaru Legacy Outback Limited, with one of those double-din head units which came with the advanced sound system (there's a sub/amp under the driver's seat.) I was originally planning on supplementing this existing setup with my carputer - and using a cassette adapter for any supplemental sound or music. However, upon disassembling the dash, and realizing how perfectly the 7" touchscreen I just got fits where the head-unit was, I'm very tempted to just get rid of the existing head unit and toss the screen in there, but I've got a few questions:

1. Is there such a thing as an adapter that would convert a 3.5mm output from a computer to that complicated plug with 274623 wires coming out of it?

2. There was another plug that looked almost like a headphone jack that I assumed was the power adapter. How much power does that give, and could I use it to power the screen, for example?

3. Would I lose sound quality by doing this? The system I have now has tweeters and a loud bass and is on par with the Bose system in my dad's new Audi. I'd assume that as long as I have a decent sound card in the computer, I wouldn't really lose audio quality.

I know this is an enormous project for a relative newbie to take on, but I'm very tempted to do so. Can anyone offer me any advice/guidance/answers regarding the above? Thanks in advance!