Thats a nasty quote. I just fitted a new turner head to my 300 130. I sourced the head and bits from Turner (about $1400) and then paid someone here to fit it up with radiator work- all up about $3500.
Now it does go better but I'd still prefer a 4bd1T. I'm hunting for the bits and when all gathered up, will organise conversion using an LT95.
If you are going down the path of an Isuzu in your defender, I'd suggest you get all the parts before approaching some workshop- unless you have deep pockets!

