I looked at it in terms of this:

Buy it for $2-2.2k.
Hand rego back in ($300??).
Good RH shift V8 trimatic (if it is rebuilt as it says), worth $400 if you need one.
Good front doors, easy worth $350 each if you had to buy them.
Good HQ guards, easily worth $200 each if you need them.
Power steer if there and complete, worth $400 easily.
Sell HQ buckets if it has some, at least $300 back.
Sell Statesman nose, at least $300 back.

All of this only worth it if you need the HQ parts of course, and if they are all good as stated (you'd want to look at them). Devilles have lots of other bells and whistles on them that are worth good money if there, like Stainless around door frames, radios, GTS fascia, consoles etc.

Of course the car may be a piece of junk and worth $1000 as said!