Andy, when you say right lens, what do you really mean though? ( not saying this as a criticism at all BTW

)
I am a big equipment whore, but I have found that beyond a certain point there is not much of a difference between the various lens. A much bigger difference comes from the type of film and the processing and printing. ( Not counting the skill level, which of course is most important). I think the real cost of high quality film photography is in buying high quality films and processing them in high end labs.
There is a difference in look and feel of the Zeiss glass and the Leica glass - I personally love the colour and contrast of Zeiss. But even then the difference is subtle, which will be easily wiped out if you were using the Tesco films and developing them in a Tesco lab.
The reason I say this is, for me, I will not drop a whole load of cash on a very expensive glass, unless I am sure I definitely want it. For that decision, you might wish to experiment with a few leica glass first. MW classic allowed me to use some of their glasses for a day or 2 ( basically to shoot a roll) and then I returned the ones I didn't want and kept the one I wanted. It was a very cheap way for me to find out, for example, that I was not being able to use Summar properly ( my fault).
Of all the Leica glasses I have used, as I said, Summicron is the very best, both for M and R version. And Chris confirmed what I had heard before - the Nokton is ( at least, almost) as good.
( Again, none of these are criticism at all - just my observations- having bought, sold and lost a ton of money chasing the equipments

)