Never done a conversion myself however I would of thought the best thing to do would be to buy a mk3 as a donor car. I'd get one with the best service history for the engine I could find and the most rust on the bodywork that way it should be cheap, I wouldn't pay more than £500 tops. If it's rusty beyond repair even better/cheaper.
You want the ABF 2.0 16v from the mk3, as far as I am aware there is not a best year they are all the same, as I said just get one with a good service history. Sorry I can't tell you exactly which bits you need.
