The brand new ones look like they could be a bit more than £400 from what I've seen, so €600 isn't too bad to start off, about £520 ish (rates are always changing, thank you Greece!). As I said they sell prowlers that are 5 years old for £350 here and the Manta range usually sell for more than the prowlers so that would work out right in my mind. The important thing is that it is worth the €600 and that the condition matches the price. They look sturdy and pretty robust most Yaks are they do take a beating no matter how careful you are its just one of those things, the guide on the link mentions the torch idea, I've heard a few people use that method. Despite the kayak being in one solid piece if the plastic wears thin on the bottom (usually the first place to go) you won't see it without a torch, and if you shine the torch through its almost impossible to not see any damage as the difference is pretty obvious.
Anyways by the sounds of it you've got yourself a good deal, hope it all goes well and you'll be joining the ever expanding yakkers out there!