Fair enough then.
Obviously you can produce cash to flash for this car which while practically irrelevant, plays to emotion, which is in your favour when it comes to negotiating.
Here is just one of many websites on how to haggle. I picked it fairly indiscriminately.
Mods on a car, especially a rare car, can be a double edged sword.
They may actually lower a cars value due to previously mentioned reasons and can include accelerated wear and tear, legality/roadworthyness issues and the ability to insure the vehicle.
Ask @k31th if it's a good idea to buy a modified car.
Also check redbook.
Of course this is an arbitrary comment but I'd start at 14k for haggling sake and see where I can go from there. I wouldn't go past 20k.
I hope this helps despite it being just my opinion.