I would opt for the RST, as you already have a N/A version of the same engine it should be fairly easy to fit.

This is the same engine as fitted to the Fiesta RST, the difference being it was EFI where as the ERST is MFI, is your current engine EFI or MFI?

This will be the cheapest 'Bangs per buck' wise out the options as the ZVH engine isn't easy and I can't really see you would gain a lot and it will still be a N/A engine, this is usually done to the bigger power RST'S, I don't know how easy the Zetec is.

If going FRST you would be best getting the management and loom all together or if yours is currently MFI you may be able to adapt to run it.

Martin