If it is a genuine factory .25 Mercury challenger then I would say it is a rare beast indeed and if shooting well worth more than £200 on its own, more with the toys.
As ever this would be subject to seeing the wee beasty as one person's view of condition often does not match anothers. Some downplay the condition of rifles, some talk it up. As is often noted good quality photos will help get a more accurate valuation. Also price depends on whether you look for a quick sale or hold out for a determined (desperate?) Buyer.
Rich.
WANTED: Next weeks winning lottery numbers :-)