From what I've seen based off the films my guess would be some kind of expensive looking suit. You could probably take a look at stills from the film for style and colours.
I dressed up as Stark for Halloween at work last year, and it was relatively simple. I got a simple LED tap light from Home Depot, and printed out a template of the arc reactor onto some thick card stock, cut it out, then taped it to the light. I got an AC/DC t-shirt (though, as Deric posted above, Black Sabbath works too), then used an elastic strap to fasten the light to my chest beneath the t-shirt, which was worn over a long-sleeve t-shirt. My job doesn't allow facial hair, but I did find an interesting alternative. I took an ordinary tea bag and emptied it into a bowl, then applied hair wax to the area around my mouth and sprinkled the ground tea leaves onto the wax. You can sculpt and "shave" it pretty easily with a butter knife. Use some more of the hair wax to get the Tony Stark hairdo and you're set.
Hey Armored Avenger, have you heard of this youtuber called Dalilomo? He just posted a video on dressing as Tony Stark for Halloween. However, it requires you to waste your phone's battery.
~Kabiko1
My husband has about at least 10 versions of Tony Stark
Cave Stark IM1
Jericho Stark IM1
Black Sabbath Stark -Avengers
Hospital Stark IM3
Falke Stark IM3
Mandarin Mansion Raid Stark/Home Depot Stark IM3
He has some versions from IM2 but I don't have any pics on my computer
Anyway this can give you a bit of a idea how to dress up as Tony Stark LOL!