All good ideas. The GPS issue might be mitigated a bit if an external antenna connector was available. Since the core PC would most likely be based on an existing retail product, I'm not sure how likely it is that much modification could be done to the existing packaging or connectors.
So I did a little research and I see that touch can be delivered over HDMI. So theoretically, if you used a BT OBDII and internal or BT GPS, you could probably get by with a single HDMI cable to the monitor. Of course, that would depend greatly on the monitor. By the way, why are BT GPS devices so expensive? I'm more than a bit puzzled.
As always, lots of variables to consider.