1.2 engine probably best for learner.
Rear axle can rust badly.
Make sure gearbox not noisy in 1st 2nd 3rd gears.
Get the youngest / best you can afford.
The Ford KA is essentially the same just slightly different shape -possibly you can get a better one for the cost of a similar fiat500.
I don't know much about the convertible but insurance likely higher and you would have make sure it works correctly! Personally I wouldn't get convertible 500