Good morning!
My widebodied Z3 is on a W plate (Feb 2000) and it has the 2.0 straight 6 cylinder engine.
As far as I know the 4 cylinder was fitted to some of the widebody models (I saw one in a garage recently) but not sure why you would want to go with the 4 cylinder over the 6 cylinder as they are worlds apart and the 6 cylinder will go on forever with makes for a much more exciting drive.
Plenty of Z3's on the market at the moment and they are certainly increasing in value so pick one up soon before the prices get out of hand.
I would recommend you look very thoroughly at the metalwork as rust is an issue with some, but by the time the widebody Z3 came in, they seem to have sorted a lot of issues.
Let us know how you get on with acquiring one and if you need any further info then just let us know.