Hello to UK,
sorry for being late with the reply.
Use the expensive NGKs. Alfa is only specifying those, they are
good for 100,000 km or 60,000 miles.
Having the timing belt changed is a good idea, but have them change the tensioner and any pulleys as well. Moreover, the
Poly-V belt is also due to change at 120,000 km (or 72k miles) and since it needs to be taken off when changing the timing belt, you may consider that one, too.
As for the gearbox, seems that a few of the 1.8l 156s do develop the problem as described by you. But it's by no means a typical problem of the 156.
I had the 156 2.5 before (while I was still in Hong Kong) and that had almost 60,000 miles (90,000 km) on the clock when I sold it, but the gearbox was as good as new. Despite the fact that I pushed it whenever I had the chance to.
I agree, having Alfas serviced isn't pure joy, and I can imagine that it's no fun at all in Britain, where everything pertaining to cars is so unbelievably expensive.
Ciao,
jaochim