Hi Filippo,
Assuming that the car was previously registered in another State, all you need is the pink slip and probably the bill of sale from the previous owner to get started.
You don't need to be in possession of the car to submit the paperwork and pay your fees (sales tax on the total amount and license fees). Don't mess around with the purchase price, they're looking for every penny and have a pretty good idea what the average cost of a replica is. If they ask, well....you are legally in possession of the car.
That's correct, they only issue 500 SB100 Certificates per year. It sounds like there are still a handful left, so if you hustle you might not have to wait until January.
Here's another good thread.
SB100 newbie question