zx3 simply means "2 door hatch", you can get a zx3 with a multitude of different engines. Example:
zx3 duratec 2.0
zx3 duratec 2.3
zx5 stands for "4 door hatch", but comes with all of the same engines as the zx3.
The SVT/ST170 engine is very similar to the zetec engine, but there are some changes that make it to where you have to buy svt/st170 specific parts for most of the mods you do.
Regarding building power with the SVT/ST170 engine, most n/a mods are very expensive and do not build alot of power. I highly recommend looking for a supercharger or turbo setup if you really want bang for your buck. Im not sure what laws are on the other side of the atlantic, but you are better suited going for boost than with trying to build N/A.