If somebody claims it's "built to the hilt", they should know every part in that motor, even if they didn't build it. You don't pay for top shelf shit and not know what's in it, even if you're paying somebody to build it.
I wouldn't be afraid of a motor just because it has had nitrous used. I would be more concerned with how long the motor has been together, how long it's been on the bottle, what kind of kit, tuneup/etc.
If he's done any serious sand drag racing, I would see if he's been to any of the sand drag tracks, and ask for times. One mans definition of fast is way different that another mans.
I can't say that I recognize the Jeep, and we've been at the sand drag tracks and sand dune drags for many many years, not that that means I know every Jeep that races. But if it's been around and it IS fast, you usually hear about it.