You shouldn't have a problem with the gearbox and the duratec as it has been used for various ford cars the past few years now, the required bell housings are out there for the v6 and I4 engines.

The V6 has been in use longer than the I4 so it may be easier to go down that route and use the 3.0 version out of a st220 for example, but you really want one from a jag as it has a better head. There is also a newer 3.5 and 3.7 version of the V6 but I don't think that has made its way into any cars over in europe yet.

If you want to go down the I4 route then the 2.3 duratec is a much better choice over the zetec as the head flows more, it doesn't take much to get 100bhp per litre out of it. Plus you can get plenty of cosworth goodies for the duratec.