pete, the spark is a nice little unit. but a "core" tube only offers an increase in clarity rather than sensitivity
personally i cant see much point in spending top doller on gen1....if the n.v bug bites you'll very soon want better kit
for a decent s/h gen1 mono have a look on ebay for a yukon nvmt with 1x lens (24mm)
i'v seen them go for 50 quid with a yukon illuminator which while not the best thing in the world is better than the onboard one
the torch's we make on here are normally 850nm wavelength which is pretty hit and miss with gen1. i'v heard of units that just cannot detect it at all!