seen the pc thing done a few times for an mp3 server. just be careful with disk access and bumps/etc. pretty easy to jog a read head and scratch a platter (or all of them)
if you were going to try to modify the factory hole for a 2 din you might get away with it. problem is there is very little room above or below the factory 1.5 din hole. the trim around the factory radio and the radio itself comes out pretty easy so you could check clearances pretty easily.
most in dash dvd players have extra inputs for things like a gps unit. go with a 1 din in-dash dvd (with an adapter for the 1.5 din hole) then hook a gps unit up to it that has an video out.
better yet, do the in dash dvd and hook your pc up to it for the gps. most video cards have an s-video out which you can throw an adapter on to make it connect right up (alot of vid cards even come with the adapter). then it would hook right up to the head unit
another option would be to get a display only overhead mount like
http://www.crutchfield.com/S-zbDZbDwGioL/cgi-bin/ProdGroup.asp?g=170450&avf=N