As the owner of an 11 year old SPF I'm not aware of many updates to current ones. There were some electrical issues with relays on early models, but most owners got free relays to correct that. As to buying a used or new SPF it's mostly whether you want specific colors and options and are willing to wait a few months or whether you might save a little money on a well sorted used one.
As Dave indicated there are few changes of substance on the GT40s. The electrical relays were changed and most all cars in the field have had the supplied relay pack installed. Some small changes in the HVAC system as well but frankly nothing that would keep me out of an early car and in fact my car is a first year build unit.
The current market makes many resale GTs very attractive as you can get into a tested, fettled car for well less than building a new unit. If the color, power train and condition are to your liking, and the price is attractive, go for it! My MK II is available: https://www.timemachineauto.com/superformance-gt40-mk-ii-p2110-1.html No real reason for selling but I seem to change GTs every couple of years.
My car is 11 years old as well. Other than having Dennis Olthoff seal around the fuel inlets (he said the early cars had issues which is why my garage always smelled like fuel) I don't think there have been many changes to the newer models. At almost 17,000 miles my car drives great. My only real complaint about the SPF design is the wheel bearing set-up which is true to the original design but not practical for a road car. But I've solved that by having Paul Whitlock machine and install crush sleeves to keep them in place.