I see the post is here since April but I'll reply anyway:
I wouldn't replace the head unit since it will probably disable the top screen as well and might look somewhat ugly comparing to the factory unit. I'm also cheap now. I'm considering getting a beefed-up rear view mirror with dash cam, back up cam, bluetooth and GPS, something like this one: http://www.ebay.com/itm/281727611153
I spent close to $2,000 with upgrades in my old car, it was Sony 701 head unit (I do not recommend it) with Sirius unit, Pioneer amplifier, Focal Polyglass speakers, Infinity Sub, back up camera, soundproofed doors and tinted front windows. I was planning to keep it forever but had to get something bigger and can't keep both cars.
What did you decide?