All parts for the K2 and k3 will work and a lot of 777 parts will work also.
I would say no upgrades are absolutely necessary unless you have the 17 degree castor blocks if so get a set of 20 or 22 degree blocks.
Some good ones to get are
King Headz, fioroni or racers edge knuckles.
King Headz, fioroni or racers edge chassis braces
I see you are in Missouri so you might contact Jim Wideman (jimjamjr) on the starting grid he use to sell K2 and k3 shock towers. I bought a set about a year ago and they are great they sure can take a beating. If he still sells them I would get a set.
Try posting in the for swap shop thread this is a link