nice looking car,
thre is a thrad about buying an alfa 156 if you search but basics are.
check the oil level in engine alfas use oil and need checking on a regular basis ,check all electrics work with the ignition on but engine not started there should be a selection of lights on the dash once started check again and there should be no lights on dash after start up in perticuler the airbag light, on start up if it sounds a little like a diesel this is probably the variator for cam timing not to seriouse as it can be changed with timing belt, timing belt which is recomended at 36000 miles so if it hasnt been done it will cost you abou £3-400, when driving listen for knocks from suspension as this is common but not expensive to fix,rust shouldnt be an issue although some cars i looked at had bubbles along the edges of roof near windscreen.
hope this helps, and as i said there are plenty of pople and threads on here to help
dont believe what people tell you about alfas thy are mostly fast fun and reliable if you look after them.