The appraiser might be a little high with his prices. Further, at the point a guy ponies up 25 large for a used Darne, he would be FAR better off starting with a new one, that had been built to fit him.
If you use your old Halifax, do go to one of the ammunition suppliers that can offer 2 1/2" low pressure 16 gauge ammunition-this is NOT the same as low brass ammunition you get from KMart. A Darne 16 with standard, American, off-the-shelf ammunition will beat the hell out of you, and the gun will be just as effective with the low pressure stuff.
Noted above, don't take the gun apart, and don't use a tool other than your finger or a plastic Bic pen end to depress the forked spring, down in the area between the rails, to remove the barrels. If you break one of the forks off, you have a broken Darne, which is pretty much worthless, and you need to get a new spring from France, or, from me. They are expensive. Don't say I didn't warn you.
For most of the production span, there was only one grade of Halifax, but, in the 1930s two grades were built. The higher of the two (No. 5, the No. 4 was the lower grade) has a bit of engraving. You will find the grade of the gun on the barrel flats.
You paid extra for a plume rib and a semi-pistol grip stock on a Halifax, so, most of them don't have that. Most of them have conventional, brazed barrels, but, I've seen a few that were monoblock construction.
Might be a good idea at this point to have a strip and clean done, along with an expert evaluation of any problems in the gun you might not see. Kirk Merrington is your man for that, here in the USofA. Google him up.
Best,
Ted