Hi,
A diesel mondeo, 2003 0nwards will do 50 plus to the gallon. don't worry about all the people that go on about servising on diesels, just have one oil change with semi synthetic oil ( and a new filter) and drive forever. Mine had done 190.000 with no oil changes since 110,000, and was still going well when i sold it.( told the man it had recently had an oil change, because it seemed like yesterday cos as you get older time flys by, so they say) diesel may be more expensive than petrol, but the benefits outweigh any costs. no spark plugs/injectors with leads etc.no electical problems in damp weather, drive it through a lake and it still keeps going, and, the power of my latest 2004 mondeo tdci is outstanding, real speed when needed, beats all the silly little boy racers in their souped up astra's and worn out bmw's. the diesel fuel lubricates the pistons/valves etc. so the engines go on for ever, unlike petrol engines that get ropey at 100,000 miles.
Thats my opinion, and, if you get a really old diesel car you can run it on a mix of boiler fuel, about half the price of diesel, but if you get caught you will go to jail, so don't get caught.
good luck,
(Can't leave my name in case the customs come round and dip my tank.)
Also, just to answer the qustion about the cheapest car, my mondeo looks lioke new and cost less than £2000. it also now drives like new after i spent £56 on new front brake discs and pads, so very cheap on parts!!!!
Alf.