All modifications need to be declared to your insurers, including alloy wheels. doesn't affect your premium though. only engine stuff and bodywork customisation seems to do that, and then only if heavily modified.
As for checking which alloys will fit, get yourself down to Halfords or similar with a tape measure and have a measure up
if you ask em in there, they'll roll the wheels out so you can have a look what they'd be like on the car.
then just go back and order em off the net!
It's what I did, although in the end, got em from Halfords cos they price matched