that mondeo wheel conversion does not look safe at all
the bbs touring car rim centres were all the same diameter, the rim sections differ in size. the 15" rims are a flat base in that there is no tyre well & you have to build the wheel into the tyre. 16, 17 & 18" rims can have the tyres directly fitted onto built wheels. There were stress cracking issues with the 18" rims and as such are no longer available. You can still get the rims in the other sizes if you have a spare 6K around