There is also a 1955 Piper Apache being advertised at the moment for £15k, which seems criminally cheap for a lovely looking aircraft, but the running costs just preclude it from being a worthwhile undertaking. (Aztecs start from about £30k as well)
You have to get your twin rating, your engine & prop maintenance is double, all your landing fees are double, the short-field/strip performance may not be brilliant and you can get some fairly basic/cheap singles that will out lift/perform them:(