So I've just recently bought a lovely mk2 oaky valver. I have lots of money spare to spend on the car and would like to know where I can put this money, engine wise to make it run better. Not necessarily performance though. What would the first things you would do to a valver? Any sensors or anything like that worth replacing after 18 years?
It's had a service this year so doesn't need one. It's going in for a head gasket/belts and clutch change in February. They haven't gone, but could do with being done

Another question.. I need to replace the heater matrix. Where can I get the best quality heater matrix from? I've heard a mk3 matrix is best - Do VW sell these? If so, how much? Do I need to replace anything else while doing the matrix change?
Thanks!