If you actually want one to run and enjoy, rather than worrying about its "investment" value, then I'd say that could be a good thing if it allows you to haggle the price down!

I would say that one is probably not worth any more than £1500 without the original case, maybe less.
Just a note to anyone who does think about buying that particular one from Rails - ask them to ensure that the loco is VERY firmly packed inside the wooden case if it is being shipped, especially if it doesn't have its original polystyrene protection inside the case - they were the ones who posted me a rather nice scratchbuilt 5906 Mallet in a wooden carrying case - the case itself was well boxed for shipping, but the packing inside the case protecting the loco was totally inadequate and the loco arrived seriously damaged, having been bounced around inside the case by the not-so-tender mercies of ParcelFarce.... I hope they might have learned their lesson from that one, since they had to give me a very substantial refund (I repaired the loco in the end), but you just don't know.....
Jon.