Rust protection was not a strong point in the 70’s and many cars from all marques suffer from rust.
You should carefully check the condition of the body as original parts can be very hard to come by. Typical rust areas include the front and rear wheel arches, the leading edge of the bonnet, the bases of the doors, around the windscreen, tailgate and jacking pads under the floor. The rear wings although looking simple are very difficult to repair due to the complicated surface design and are not generally not available as a replacement panel.
The steering on an SM takes a bit of getting used to, and as a result a number of cars may have had front end events. Front wings can be quite easily repaired but the front headlamp assembly on the European specification cars is a different matter. The stainless steel bumpers are very difficult to work on and the plastic headlamp carriers are not available.
The glass sections covering the headlamps and numberplate are occasionally available as new or used items, but are expensive. It is wise to fit poly-carbonate replica sections. These are extremely convincing and virtually bullet proof.
Small trim pieces are notoriously difficult to find so check that it is all in place and not damaged, and if removing the windscreen or rear window for a respray or replacement of the windscreen the stainless steel trim can very easily be damaged.
The main body structure is very strong and although it can rust it is real;actively easy to repair. The undertray is smooth (good for the aerodynamics) which also assists if sections need to be replaced.
You should check the front body sections where the suspension is connected to the longerons as these bolts can come loose and cracks can sometimes be found in this area on early cars, although are easily welded if the engine is removed.