40p a mile working on an average of 3k a month means £1200 quid eek allow 400 a month for fuel (3000/40(allow for a bit of abuse in the mpg) means around 75 gallons of fuel a month
which leaves you a stonking 800 a month for a car,
or a budget of circa 5 grand. buy something at a year old run for 6 months and batter the miles on and then sell it on and take a couple of grand hit
result you score a couple of G's