Re: Your opinion on 84 3/4 ton.
If I recall properly, my 1984 3/4 ton came stock with a 305, TH350/NP203, 14 bolt SF rear, GM corporate 10 bolt front with 3.42 gears and ~31" highway tires. Stock, it wouldn't have made much of an off road truck, even with better tires. It's a much better truck now that I dropped in a built 350, SM465/NP205, 14 bolt FF, Dana 60 and run 4.11 gears with 35's.
What you are looking at doing WITH this truck has a major bearing on what you will have to do TO the truck. You'll eventually want something more than a 305 for off-roading, in my opinion.
$3000 for an `84 3/4 ton sounds fair IF the truck is in pretty decent shape, runs & drives good AND has a decent plow on it. As already mentioned, you can sell the plow or use it. I sold an OLD Meyer plow a couple years ago for $650 if memory serves... and it was pretty beat up. Western plows are more popular... you could probably get $1000 or more if it's complete, in good condition and functional.
I have been plowing snow with older Chevy's for at least 6-7 years. My first 3 or 4 years, I ran a 1982 K5 with a 6.2L Diesel and TH700R4. I never had a problem with the transmission. The last 3 years, I've been running a 1977 shortbox with a 305 and TH350 and haven't had a problem with that tranny either. BUT both trucks had/have an extra tranny fluid cooler. Check the tranny fluid in the transmission - if it brown and smells like burnt rubber, the tranny is on its last legs. If it's still red, it's probably fine.