Need advice on 1918 artillery

[Dave Sanders]

I have a chance to get a 1918 artillery luger with holster and stock. I don't have pictures, so I know it is hard to give an estimate, but I wondered value range. The gun is all matching inside and out. The bluing rates at least 90%+. I didn't take the grips off to see if the grips were numbered as the screws were both really tight to turn and didn't want to mess with that. It has a period nice shape holster with original cleaning rod and unmarked tool. It has an original shoulder stock, but the number doesn't match the gun. There is also an imperial proofmark on the stock. The screws attaching the board look old and unturned. The board itself is in nice shape with no chips or bad scars. It looks like a nice rig.. The mag was not matching to the gun, but original period mag. The guy is taking offers, so I offered $3500. I wondered if that is a fair offer, or am I too high or low? Thanks fo9r any advice you may offer.
Dave