Minecraft with a voucher of your choice?

I have a question,

At e.g. Rewe there's the so-called wish voucher, you can redeem it on the Minecraft website, I'm just not sure whether this is possible, and I do not want to buy a wish voucher unnecessarily.

You can "load" the money onto your PC, can you then buy Minecraft on the website?

So at mojang itself you can't redeem the voucher, but take a look at the wish voucher website, there are all the online shops where you can redeem it.