...Will a 430ex flash be strong enough to light up the whole bulding considering the distance? I'll be using a Sigma 18-200 os len.
My guess is that the flash won't be strong enough but maybe the expert flashers can fill in the detail. If you have, or can borrow, a tripod take it along as well. That will allow you to keep the ISO, and so picture noise, down but the long exposure times may be an issue if there are people or vehicles passing and you aren't looking to introduce motion blur into the composition.