I am not in the trade, merely speaking from personal experience as I tend to run old, very cheap cars.
All my cars produce �mayonnaising� because I do very short runs most of the time. It doesn�t necessarily indicate any engine problem at all.
As Teddio said, when buying a car I always check the dipstick and the coolant for any sign of contamination of the oil or coolant. You don�t want to see any rust-coloured deposits in the coolant reservoir, or rusty streaks running down the engine block (these can be difficult to spot on some cars), which both suggest a dodgy head gasket. A low coolant level can also be indicative.
I did buy a cheap Polo not long ago which turned out to be a dog in several ways. I didn�t detect anything wrong with it when I bought it, and it ran OK for a few months, but I wouldn�t buy another one of those!
Some engines are more prone to this than others, but I don�t know which they are.
I�ve found Fiestas to be very good usually. Got an Escort now, which has been great.