If you can afford the turbo setup go with it,I'm not a fan of temporary power and a sleeper is a car that beats cars much quicker than it's looks and lineage suggest and a NA setup will still be slow when off the laughing gas.
If I could afford an ST I might be up for swapping over the nose and tail parts from a regular MK3 SE which would be an awesome sleeper.
'12 Focus Red Candy SE Sport 5 spd
'99 Toyota 4Runner Sport
'11 Subaru WRX STI Hatch