Nothing wrong with a 1 series, they drive very well, you might just be able to stretch to a 130i in that budget. Not sure about a Z4 though. You want the 3L if you do, as there isn't any running cost disadvantage over the smaller 6cyl engines.
If you're spending this much on a car, you don't want a Fiat. I think from this shortlist I'd definitely want a BMW. And I'd probably look for a 330i coupe or saloon But they might be a bit big
S2000s are ace, if you're a fan of the revvy power delivery
Autotrader - BMW Z4 3.0i 2dr Roadster FTW , seems theyr'e in budget definitely a cut above the "standard" hatchbacks for this money