Can you actually tell if a comet is pristine or not?
If you want a pristine frozen piece of the Oort cloud, shouldn't your mission go out there to get said piece? Otherwise, aren't you risking that your "pristine" comet has actually made numerous trips into the inner solar system, but just not in a timeframe or with a level of ostentatious display that was detectable by modern science?
Obviously, getting out to the Oort cloud means a tremendously long journey, but NASA's New Horizon probe shows that it can be done and that great science can be retrieved by a craft at that distance from the Earth. You'd have to work out how to get a probe out there at speed, and still be able to orbit or land on a proto-comet, which sounds like no mean trick. However, I would think that would be the only way to make sure you have a pristine sample instead of one that has been compromised by one or more run-ins with the Sun.