DN2 and DN3 are both avaliable from PBW. DN2 is much closer to stock SE4 except for a whole lot more vehicle/unit sizes (65 standard shipsizes alone). Also, DN2 has many Mounts.
DN3 has the same shipsizes but they are each hull type has it's own research tree (carriers, standard, colony, transports) and also adds a Tugboat shipsize.
Also, you'll find that in DN2 the research cost for ships is normal so it ends up costing more to develope destroyers than it takes to develope warp openers
Hence the changes to DN3.
If you are looking for Dark Nova 1, you won't find it. There isn't one. The original Dark Nova game was a stock SE4 game. It was the colletive work of the players in that game that started the mod.