I fill mine once a month then top up in between if it gets a bit low probably averaging about 19 mpg around town but that can drop if my right foot gets a bit heavy, on motorways I'd estimate about 28-30 mpg but I don't often use motorways
Hi &
Check overall condition, as long as service history is good and it's been looked after mileage shouldn't be a problem, tbh I wouldn't worry too much about mpg it's a 3.5l v6 so it's never going to be economical. Good luck with your viewing