the vt800's are great bikes (but I'm biased) - I'd: change the oil and filter, bleed the clutch and brake, lube the choke and throttle cables, change the final drive oil, change the fork oil - in that order of importance. You could also flush the coolant.
Also inspect the tires - like you said, it's likely they are somewhat cracked/dry rotted. If its sat long enough, the carbs may be gummed up and need to be removed and cleaned, but if its just running a little rough, try throwing some seafoam in the tank at each fill up during the season this year. Also manually inspect hoses and rubber for cracks and the tank for rust.
How long did it sit and did it sit inside or out?
Like GWS said, they are almost identical to the 86-87 vt700's, except for the 4 speed transmission, 100 extra cc's, some cosmetic differences, and a few other minor things.
They usually go for a decent amount more than their vt700 bretheren, because they were only made one year. In the last year, I've seen 3 really clean stock examples sell on ebay for 3,700, 4,300, and 5,700 (belive it or not!). Average good stock condition usually nets between 2,500 and 3,500.
If you have any other questions, feel free to ask me.