I think this is an awesome idea. Overall the prices for parts seem very inexpensive. For example, fox air shocks will cost a minimum of 200 each for new. Please don't skimp of the safety equipment. I think you should figure at least 250 for both harnesses at a minimum. Sheet metal is cheaper than rhino plastic, but plastic won't need paint plus you might be able to pick up a used set off of ebay for cheap.
You didn't add any pats for a radiator or cooling.
pulling your chain here.
Overall the pricing seems about 30-40% or more low. No matter how hard you try to plan everything out to the smallest detail you will always forget something.
I think planning to spend 8-9000 on a buggy is a nice target, but don't let your brother get sticker shock when the price comes in between 10-12000.
Go TEAM TUT!