Conducted a post-mortem on the gearbox, dry as bone inside and the teeth on both bevel gears are cactus. Doubt it had ever been greased since new.
The rest of the machine showed evidence of severe neglect - air and fuel filters gunked, black oil, spark guard sooted up, no sign of ever being cleaned, etc.
On the good side though it came with all the original accessories in the red Honda bag and appears the metal blade had only been used briefly so I suspect it's done lots of time trimming rather than heavy duty brush cutting.
After doing a complete TLC job it is running nicely.
Great to have two machines again, saves time being able to do a bit of trimming then grab the other machine for cutting nasty stuff and then back to trimming and so on without having to change the heads.
I don't mind if people neglect their tools and I get them free or cheap when they don't start or run properly and just need minimal work to be revived. I had planned on spending $800 for a new 435, don't see decent second hand ones available often.