Firstly welcome to BMWOC  All the versions of 1er you mentioned all home the exact 1995cc N47D20 engine.  118d - 147bhp 220lb-ft 120d - 170bhp 260lb-ft 123d - 204bhp 300lb-ft  The only differences are technical re-maps, inlets and I think turbos.  The road tax on the 118d is £30 a year if I remember right, but still quite cheap on the other 2, less than £100 I think.  £11000 is a bit excessive for say a 2010 model, I'd be looking at spending 9-9.5k max  Also, you say sub 60k miles...honestly mate, this engine is bullet proof...save yourself a few K and find a nice M-sport with 100k and buy some real nice wheels.  Hope this helps you decision dude.