I think you'll find it hard to actually "buy for"anybody. If it's for export you will need to have people 'in the trade' involved anyway, so your role would pretty much be reduced to inspecting cars on behalf of the prospective buyer and letting the professionals do their stuff.
Japan is one of those places where short-circuiting the system doesn't really work. You'll most likely be on a three month Tourist visa (extendable in-country, but there's a limit...) and therefore won't have much time/status to get a properly rented place of residence and all the other gubbins you need to own and run a car, let alone start any business activities (necessitating change of visa status...). Renting a car of course is easy, but you'll need somewhere to park it legally whether you have it for one day or one month.
I don't want to come off as too negative (probably too late....) but Japan's bureaucracy tends to win in the end. A bit of casual trading in parts is one thing, but whole vehicles is another thing altogether.