you can often pick them up with nearly new tyres for less than the cost of new tyres for the claytons making the alloy upgrade effectively free.
This is both a ridiculous and brilliant point at the same time. If you need new tyres for your 16" it's a no brainer. Even if you already have 17's, it could still be cheaper to get a new set of wheels.
I have seen a few people talking about spacers, is it ' normal ' to have 25 on the back and less on the front ?
Yes. The front wheels sit further out/ the body is narrower, so the rears need more spacing to look the same.