Hi Iceman - Without question I would say to go for a genuine Naim hicap. If you use anything else then you will invalidate your warranty it nothing else. If you want to try second hand then look at Tom Tom Audio - they have an excellent range of second hand gear and are very well respected Naim dealers.
Most people who know what they are talking about say that the differences (spec and performance) between the olive hicap and hicap2 are pretty small. However remember that to perform at it's best the hicap needs to be recapped every 10 years, so factor that in to the cost if you buy an older one.
Give Naim a call to find out what a recap costs (or to find out the service history of any second hand hicap you may be looking at) - the Naim forum is also a good place to ask these sorts of questions.
Whatever you go for i'm sure you'll enjoy it - adding a hicap(2) to my Supernait made a really nice improvement.
cheers