If you want to go for a very easy, lower power solution you can always buy a mac mini (~50 watts depending on usage) with a 12v power supply and a 12v screen.
Mac mini power supply (make sure to get mac cable kit)
1. Motherboards - All of the motherboards we stock will perform these basic functions.
2. Power Supply - This question is highly dependent on what case you get. You definitely should get some type of regulated power supply. I assume your system is 12v. The M2-atx Power supply will filter power from 6 to 24V.
3. All of our screens will work. 12v screens will avoid the use of an inverter. For best screen quality and longevity, power your screens off of filtered power.
If I was putting one in my friends boat I might recommend:
30-45 watt Intel board - http://store.mp3car.com/product_p/mtb-057.htm
small case - http://store.mp3car.com/Morex_3677_M..._p/enc-022.htm
external optical drive (use as needed to save power)
More usb ports (avoid using hubs)
You would then need to pick a few small other parts like cables, hard drive and memory depending on your needs/budget.
Hopefully this will give you some ideas to get started. Welcome to the forums.