The 78F has some challenges, I would look for up to 76K.
The main problem with 78F is the head, the guides tend to get loose around 20 -30000 miles and they need to be replaced together with valves, retainers and springs. How do I know? I have F3 engine on my bench waiting for the cash to continue. Check out CycleX for their F special area.
On the other hand, 78F has dual front disc, disc break in the rear, you could find wheels that are made to accept tubeless tires, i heard the transmission is stronger than early Ks.