The 6.9 L diesel engine was discontinued in 1988, to be replaced by a 7.3 L diesel made by International Harvester.
One major problem this generation had was rusting. It's hard to find one of these trucks these days without rusted out wheel wells in the truck bed.
This generation also saw the introduction of the F-Super Duty. These were considered a Heavy Duty F-350, or a F-450, depending on who you asked. These were industrial strength trucks, and they were mostly bought for commercial use in things like wreckers and tow trucks.
These trucks are readily available on the used market. Like I said, there were plenty of rust problems, so look for one with a body in good condition. You'll be best off buying one from southern parts of the United States, where they would have been less likely to be exposed to road salts. Check on eBay for listings from across the US.