I have a V40 cc on 18” rims and they are always buckling because of our poor roads. Will xc60 rims fit and are they any stronger?
I can get hold of some 17” xc which already will help with sidewall height.

 

Yes you can fit rims from other Volvo models - assuming they have the same center bore (S/V 40s are 63mm for the most part, the bigger cars went to 63 in about 2009) and a similar offset (varies by rim width but FWD/AWD tend to be in the 40-50 mm range). Check out wheel-size.com for specs by model/year.

A better question is what tire pressure are you running? If you are flattening rims you should add 3 PSI or so to make the side walls firmer to protect the rims, I generally run 36-39 PSI on my car with 235/45-17s).