Wiring gauge?

[Grapes]

I would like to replace some wiring, mainly for the electric windows but since I'm buying wiring anyway I might as well get some extra in. I was told I should make sure it's the correct gauge and I haven't been able to find out what gauge(s) are used...
Also I noticed some available options are plain copper and others are tin plated or something? Is there any advantage or disadvantage there?

[Anders Dinsen]

I generally use 1.5 sqmm og 2.0 sqmm plain copper depending on the load, multi stranded wire, e.g. https://www.vehiclewiringproducts.co.uk/c-48-cables/c-198-single-core-pvc-thin-wall-cable/p-975-25-amp-2mm-sq-28-0-30