Don’t know how old an engine it is and whether you’d find something within budget that has it, but I’ve been impressed with how lean the VW 1.4 tsi in my Audi is.
Used to have a diesel and would get high 500s, maybe 600 miles from a 45 litre tank in the summer. Get between around 550 from a 50 litre tank in this. Factor in the difference in price between petrol and diesel and its probably not much difference.
I’m talking from fairly limited experience (2 of them, to be precise) of them, but reckon VW petrol engines across the board might be worth looking at. My wife’s Ibiza has the 1.0 tsi and when I use it for work it’ll easily get 70mpg.
Disclaimer: I could well be talking shite and older ones are tat.