Please don't take this the wrong way but if you're worried about gas mileage then I think a Ferrari may be the wrong choice. If you can't afford the petrol, you can certainly not afford the maintenance. A Mondial is cheap to buy but it's still a Ferrari and you need to budget at least $3k a year for maintenance. FWIW, you should be looking at around 18mpg.
I would not drive an '80ies Ferrari on a daily basis, although there are people that do (fellow member Schumacher is one, I seem to remember). Ferraris certainly benefit from frequent use, but it's like using any other 25 year old car as a daily driver. Things will need to get fixed on occassion, and this might happen just as you're late for a meeting.
Onno