If you really wanna save money, then I'd be looking for a second hand TDI of some description for less than the price you get.
Doing what you are planning is not going to make a huge difference TBH.
A VW Lupo 1.4 TDI Sport / Seat Arosa 1.4 TDI Sport will both do 70 MPG on a run. OK its a diesel, but it can also be chipped to 100 bhp and 180 lb ft torque - decent performance, group 4 insurance, 70 MPG!!! Cheap to buy and run, probably not been thrashed. You'd have change from the amount you get for your car. These cars will have done most of their depreciating. You are proposing to buy a car, which may not be that much better on fuel, and WILL depreciate more than a 182 in the longer term.
Your idea makes no sense. Buy a cheap VAG diesel, like the one I said, and it will make a BIG difference. Thats what I'd do if I were you.
I'm in a kinda similar situation but I do around 200 - 250 miles a week max. Also I used to waste a lot of money - going to KFC or Burger King at dinnertimes - started bringing my own food/drink/sweeties in. Much cheaper. £70 a month phone bill! f**k me that is a lot. Mine is about a fiver.