I don’t think that it is possible to get a definitive answer to, “How much work could I get done for $20,000?”, due to the variations in pricing from one area of the country to another, and due to the variations in pricing from, “artisan”, restoration firms to, “Mom & Pop”, operations.
This is sort of like asking, “How much should a suit of clothes cost me”. You can buy a suit at J.C. Penney for…let’s say…$120.00, or you could buy a suit at Brooks Brothers for…let’s say…$600.00, and there would be stores that sell suits for prices in between those two extremes. They will all be very different in terms of the quality of the material and the workmanship, but…they will all be suits.
I think that your best bet is to find the auto restorers in your neck of the woods, and have each of them give you an estimate on restoring the specific parts that are most important to you. Bear in mind that they may suggest things that you hadn’t even thought of, as restoring one part frequently means restoring other (less visible) parts.
As to converting it to an SS model, just bear in mind that this trim variation will conflict with the information in the car’s serial number, thus making it less valuable if one of your descendants decides to sell it at a later date. A totally authentic restored vehicle will be worth more than one that was, “upgraded”, after the fact in order to make it appear to be something that it wasn’t originally.