Buying FE 390 2011

I have a 2011 FE390 and LOVE it. Mine has about 300 hours (over 200 when I bought it). These engines do NOT need the internal engine work as you mentioned. I know 3 people who have had 4 of the FE390s. None of them did any internal engine work for hundreds of hours. Some put several thousand miles on these engines with no internal work at all. Keep the valves adjusted (but if you are not racing, even that is not often. Mine have not changed in 300 hours).
IF you are not racing AND are NOT feathering the clutch often, then 1,000 miles on an oil change is not a problem. You probably know that engine oil circulates around the clutch plates, so small clutch particles get in the oil. I feather the clutch very little, because the engine is has good torque and I seldom need to feather it for slow speeds in low gear. One of my FE390 owner friends owed a major motorcycle dealership for many years and has had two of the FE390s. He told me personally that 1000 miles on engine oil is no problem IF I am not racing and I don't feather the clutch much under power. I use the clutch gently for normal RPM gear changes, but often change gears without the clutch IF the engine RPM is right (no load).
SO... the type of riding you do will determine oil changes. I am now changing my oil once per year - preferably at the end of the season - no winter riding where I live - so fresh oil sets in the bike all winter, I ride all summer (6 months or so) then change oil for winter storage). I ride less that 1,000 miles per year and it is going great.
BE SURE your bike has the newer fuel pump - older type stops woking at higher temps. Also use "Engine Ice" or other coolant additive - these bike run hot and need both the additive AND AN AFTERMARKET RADIATOR FAN. Some owners install a heat blanket under the fuel tank (I boiled my gasoline once on a long, steep hill at 95 F outside temperature).
You won't regret buying this bike, if it is in good shape and esp. since it has not been raced. I am 70 years old and still ride forest service roads, high mountain passes and some off road in SW Colorado. We did 436 miles of the TransAmerica Trail in Sept. 2019. Both I and son-in-law on FE390. It is the PERFECT bike for what we do.
Husageezer - Durango, CO area

Oh, I forgot to mention the clutch. ALL of these bikes have "sticky" clutches. It get MUCH better when the engine reaches full operating temp. ALSO, the gear changes are easy, but it is hard to find neutral with the engine running. We simply kill the bike to put it in neutral, but it gets easier with practice (light touch to find neutral). When cold, start the bike in neutral, then give it just a bit of throttle to go to low. Truly no problem when you get used to it. But it DOES want to move when cold.
ALSO, we LOVE the Lithium Ion batteries. That's all we use now.
Husageezer

The clutch issue could be the start of a slave cylinder seal failing. Easy fix but does tend to happen to alot of them.