I purchased a 6th gen Celica, my first Celica, with rust underneath, but I made sure it was just surface rust that can be sanded, or sandblasted off, or even just painted over with a rust converter/inhibitor, which I did before the winter months came. So far so good.
If the exhaust is the only thing not rusted, determine if it was replaced or if it was the original, or how old it is. Just check and be sure there are no holes or cracks in the cross members or other structural areas, you really have to look and if you're taking it to a shop have it put on a lift. To me orange color means it just started oxidizing at the surface, once it started turning red or brown then it has gotten deeper, but I'm no chemist, you really have to look if the rust is uniform all around, or just in certain spots.
If you do get a car with rust on it, and treat it as I did, realize you'll have to do it every year, unless you totally sandblast it off and re-primer and paint, then you won't have to do it for a while, but as with any metal or steel, and depending on where you live, it will come back again, just depends on how much you want to deal with it, versus a "rust-free" car which you won't have to worry about for a while, but again depends on where you live and how well you keep the corrosive stuff like salt off of it.