I have a Beseler Slide King, and they do project a beautiful image, though I haven't tried projecting on a surface as large as the side of the building. The one I have was used in an auditorium. There are wider lenses, but they're hard to find. I've picked up a lens or two and some slide carriers from a place that specialized in them. The projectors are usually fairly cheap on eBay, but the accessories can be a little pricey in comparison.
Bear in mind that you may have to make custom mounts for your slides with a Slide King, since carriers for standard sized mounts can be hard to find, but once you've narrowed down the images in your show, it's not so hard. You can cut them from mat board like little window mattes.
It is also a totally manual projector like in old movies--insert slide, move carrier, remove old slide, insert new slide, and then move the carrier back to the previous position.