rx8 won't idle without gassing.

I'm looking to buy an rx8 that just came up for sale but it has a problem starting and then can't idle without applying gas. The owner says he's changed the plugs and all that. He also says it has white smoke but it goes away after a second. Doesn't overheat or anything like that. He believes it has a carbon buildup in the intake but I was wanting an idea of how much that'd cost to fix it or if there were any other opinions.