How efficient does it need to be? As in, how heavy is your right foot?
For example, my diesel gets about 43mpg but it's still cheaper for me to drive the MX5 which only gets about 35mpg, simply because petrol is waaaaaay cheaper than diesel. Plus the petrol seems to care less how I drive it, when the diesel is massively sensitive to right-footage. No modern diesel will ever get near the claimed 60mpg but published petrol figures tend to be more accurate, so worth taking that into consideration. I'd be looking at a 1st gen 130i personally, they're a cracking engine and a really good drive with parts easily available.