Nothing wrong with a 400 Chevy. They were always a "torque" engine - just the thing for a truck, especially a 1-ton
Only thing to watch is, they're more sensitive to overheating because of the block design: To cram that many cubes into a small-block package, adjoining cylinders are "siamesed" (no water jacket between the bores)
Remember too that the 400 is "externally balanced" - not enough room in the crankcase to add sufficient counterweight to balance all the moving stuff, so the flywheel/flexplate & harmonic balancer are inherently "unbalanced" and unique to the 400 - can't use ones off a 305/350/283 etc on your 400.
Your truck's stock cooling system, properly maintained, should be fine. What I meant by "sensitive to overheating" is if the engine gets mucho hot as a result of a fan belt breaking, rad/heater hose letting go or general cooling system neglect, the 400 is more likely to sustain some damage than a 350/305 etc. Regardless of what engine you have, overheating is bad news. And if your truck doesn't have a temp gauge in it, put one in.
Without throwing TOO many $$$ at your engine, one possibility for an upgrade would be something along the lines of Edelbrock's "Performer" intake manifold & a cam ground for low & mid-range power, along with a good dual exhaust system.
Sorry I can't help much on the price question - I'm from Canada (eh !) but something that old, best to check it out thoroughly first - both condition and history if possible. Has the truck been plowing, was it a company (read - "work" truck, after seeing how some of my co-workers treat our welding trucks I'm not sure I'd want to buy anything they'd been driving...........) and what has the maintenance been like?) My old '75 doesn't have a whole lot of the original 1975 issue stuff remaining (did a major overhaul on it in '96, plus I'm always tinkering away at something on it)
Going "out on a limb" here, $2500 US works out close to $4000 Canadian at current exchange rates. Being 4x4 and 1-ton to boot, providing the truck checks out OK as described above, it may be worth going after. I had mine appraised in '97, 1 year after the overhaul, and it was valued at around $7000 then. (Then, not now - what 5 Ontario winters can do.......... - time for another "refit" soon!)
$2500 for a K30 is a pretty good deal.
I have seen the axles go for $1200 here.
A D60 up front and 14ff in the rear and most likely 4:56 gearing.
If the rig is in good shape then that price is a good one.
With regard to your cooling questions/concerns, one additive that works quite well is manufactured by Red Line Oil is called Water Wetter and is available in both regular and diesel formulas.
It is compatible with both plain water and glycol-based anti-freezes/coolants. Many race tracks allow its use because they have disallowed glyclol-based coolants in the interest of safety (glycol is super-slippery on asphalt racing services).
Many times, a 30 degree F decrease in temperature can be realized.
i just got back into town but when i went to look at the truck a little while ago i decided to buy it. i will be picking it up the middle of next week. then i am going to tear it down and do a frame up and re-body it with the fiberglass body. if you have any questions let me know i would love to talk about it.