For optimum roadholding a 165 section tyre is arguably best, but for the pure handling and retro feel a 145 is great. A 145 makes a Mini feel the way the original classic Mini used to feel and handle.
With a 145 the steering geometry is 'as designed' and the tyre contract pressure onto the road is just right.
Remember, originally the Mini came with a 5.20 x 10 crossply. Fortunately radials soon replaced the crossplies, but my first 850 came with Dunlop C41 crossplies.
Is the difference between 145s on a 3.5" rim and 145s on a 4.5" rim very noticeable?