You might want to put the wider tyres at the rears.
Like what JnJ said, it'd be stretched, with 18 x 10.5 so you might want to go at least a 255.
Preferably run all same size tyres all round.
50% is fine, as long as you are not down to the indicator mark.
But I would be buying new tyres anyway.
Shop around and compare prices.
Tyrepower Hawthorn / North Melbourne and On The Run Motorsport can all do good prices on high quality tyres.