I have a GP3500 I would be willing to let go for a reasonable price. It's in my shop right now so my information on it is a little sketchy. It has the Coiltek Li-ion regulated battery system, with 2 batteries and 110v charger for them. (Bought this battery system instead of the ML battery setup as the Coiltek was the "hot" setup when I bought the detector. I bought the detector new shortly after the GPX 4000 came out. I have two coils for it, the stock 12" DD ( I think it's 12") and one other one an elliptical I believe and I think it's a DD too I'm pretty sure it's a Coiltek, but not at all sure about the info on it. I have the ML carry bag for it. Not sure about the box and the instructions but if you show reasonable interest I will try to find that stuff. I also have/had one or two JP DVD's on this model. I'll get more information on the unit and the extra coil this afternoon when I go to the shop to get the RZR to fetch the mail. The extra coil also has an attached rod so it would basically be a plug and play deal. If I get a chance I'll either make a new post or edit this one with additional information on the extra coil early this evening.
Let me know you interest level especially on the DVD's and Instructions - they will be the most difficult to find. I have 9 or 10 boxes of stuff to go through to maybe find that stuff.
If you want to contact me my email is MikeT124@protonmail.com I check email first thing every morning, but generally only once a day.
Mike