Not that I own, or am ever likely to own, a Purdey but if you're leaning more towards an "investment" purchase, I would think it's a bad idea. IMO true investment guns are absolutely 100% unmessed with or those with some kind of unique provenance. That goes for Purdey, Parkers, Foxs, or any other make of gun.

If you're looking for a shooter that will hold it's value, it may not be a bad purchase. Short of major damages or really bizarre alterations, I can't imagine a Purdey would decrease much from reasonable use. The board has given good advice here and I would tend to agree with them.