Ok, so you will be ending up with a setup very similar to myself, in that I run a MacMini, but on Windows natively and Centrafuse as my front end. P1900 will be likely more than enough, the one I got was overkill, I think they offer very similar features these days. Best place to locate it is behind the passenger seat in the cubby. There is a fake floor which you can put it under. You can also run the power cables down the passenger sill from the battery via a large grommit which is by the battery (told you it was simple!). I have my Mac itself located in the passenger side "Letterbox" cubby between the rear speakers.
For GPS I recommend the Globalsat BU353 USB. Can be had from Linitx along with the screen and PSU. This can be routed out the back of the cubby and under the rear strut pastic to give good reception and also hidden from sight. Dont bother with the Centrafuse nav edition, its pants (until V3 comes out anyway). Loads of us are using iGo 8 for PC which I can set you up with if you like (when you are ready for it).
That should get you going