no, the body is a great ground and will work fine. I used the seat bolt for mine.
The other smaller ground is for the inverter case since the main ground is only grounding the internal circuits. Just connect it to the main grounding on the inverter.
really depends on what you running I run 2 awg for my 1000w and I just have to start the car for higher draws to keep the volts up.
Im a fan of Xantrex and would recommend the 1000w model so You can run some higher loads if needed. ive run my little coffee machine(600w) a handful of times.
http://www.amazon.com/Xantrex-806-1...e=UTF8&qid=1426557092&sr=1-1&keywords=xantrex
or the 600w model
http://www.amazon.com/Xantrex-806-1...e=UTF8&qid=1426557092&sr=1-4&keywords=xantrex
Look at my build for some idea's with the inverter
How are you liking it? I have the 1000w waiting to get installed. I was thinking of mounting it under the drivers seat. Where did you mount yours?