GT TDI 150
if you read my post
here it shows the savings on fuel on the GT TDI over the 1.8T
i get 50~55 mpg from my TDI 150
a TDI will cost more to buy than a 1.8T but if you're doing the miles you can easily make that back in the fuel savings. i think the TDI lumps are meant to be less problem prone than the petrols too.
both the 1.8T and GT TDI 150 can be chipped to nigh on 200 bhp except the TDI gets what matters and thats torque.
the TDI is ultimately limited to about 270 after a new turbo, where the 1.8T can go to
if you're that worried about cost you could consider an LPG conversion which means you can keep the petrol lump but unless you're doing something like 20k+ its going to be more than a TDI