Unfortunately, NK Pro has never got the support, from either publisher or joe public, it deserved. However one league keeps on battling against the tide and can be found here
http://forum.gpchampionship.com/ . I can highly recommend it as a league and I still enjoy NK. If your after single player fun forget it unless you like hotlapping, no NPC I'm afraid.
As for single seat racing I couldn't recommend RF yet. Other mods are good (Megane, caterhams, etc) but the single seater mods, so far, lack something to me. For me, they don't get close to NK