Can anyone out there give me some sound advice?

I run a standard 1.8 zetec 54 plate focus estate which I have put a body it on, got some decent wheels, lowered suspension, and tinted the windows.

I am now looking to upgrade my engine so that I have performance to match the looks of the car ( I know most won't like the look of a lowrider estate but I think it works )

What is the best engine to upgrade to? Is it the RS or the ST? What are the power differences and the problems that each conversion will throw at me?

I have a garage, a bit of cash and plenty of time.

Any help or advice would be brilliant as this seems to be the place to get it!!