I would definitely start with The Colour of Magic if I were you and work your way through the series in order. It's how I started and I think it was well worth it.
It is sad to learn that he has developed this condition, but it can take many years for it to start advancing so hopefully he still has many years of writing and enjoyment to go before it can't go on any longer. Plus he has already written a lot of books in the discworld series, so anyone just starting out with the first book has a lot of great books to read already.