Check with the RV crowd - they have been doing 12 volt lighting and a bunch of electrical based on what you are thinking for years. If you have a big RV store close, they will have lots of 12 volt fixtures, lights, wires etc. You will also find a lot of good information. Be nice to the service guys and they can give you a bunch of tips too!

There are also some very good reference books on 12 volt electrical in RV supply stores and also marine supply stores for the bigger boats.

Paul -