If your thinking of buying an old classic mini below are some very basic pointers of things to look for.
The list would of course be endless but below are very common places to look.
A Panel (The A Shaped panel in front of the door and behind the wing)
The likelyhood that these have already been replaced once if not more is very high on older minis. If they have checked how the door lines up against them, is the gap even all the way up? Any sign on bubbling of paintwork indicators rot underneath and the only way is to fit a new panel. These are not expensive, But is the labour has to be added it isn't cheap.
The Windscreen Scuttle panel - Very prone to rust, Look for bubbling of paintwork where a bodged (fillered) repair have have already taken place. The only way to fix this properley is the Windscreen out and a complete new scuttle panel installed. The Panel alone is not hugeley expensive at around 60.00 but if you have to pay someone to do it, the hours alone make it a very expensive job at around 180-250 pounds.
Sills - These always rot but luckily they are fairly inexpensive and any mini specialist / welder can fit these quite reasonbly.
Floors - Lift up all the Carpet, Mats aswell as looking from the uderneath. Bodged cars can often look like a patchwork quilt or repaired welded on pieces of metal.
Door Bottoms - Look for any tell tale signs of bubbling at the bottom outside of the doors and Also at the door bottoms as these can be prone to rusting. Used doors can still be obtained cheaply.