I love the stock instruments on the S2000, its like something out of star trek :-D.
Personally, I would replace the stock radio. I never understood why people would keep the stock radio when they have a Car PC, but that's just me. Your idea about putting it on the passenger side is good too. Here is a pict hat might be helpful:
Also, what do you think about putting in something that would pop out of the center arm-rest storage area? This would be a bit more stealthy also.
Another option would be to mount a screen up on the dash, but this wouldn't look as clean, IMO.
I'll be following your thread with this one. Good luck.