this is just my personal opinion, but i think 18s are too big for a Mk1.
I like 17" for looks. and the sidewalls would cushion just a bit more. thirdly the tires are significantly cheaper. fourthly, 17 wheels would be lighter.
May 2014 ROTM
for auto-x: '04 Liquid Grey ZX3
for street: '02 Infrared SVT