Well it's certainly debateable. IMO, I'd be quite skeptical and look at it from all aspects. 180K miles in 4ish years is A LOT of driving!!! While mileage isn't necessarily a bad thing... a good maintenance schedule for a truck like that is extremely important.
Some questions I'd ask are...
Does it have or had any mods?
Was it a DD or a heavy hauler?
Does the owner have a maintenance spreadsheet on it?
Any major parts replaced? (which a spreadsheet should cover but it doesn't hurt to ask)
Original owner?
Any rot?
Aside from that, take it for a good drive. Listen, listen, and listen some more for anything that might sound out of wack. Let the engine get up to temperature, along with the transmission. Also see how it feels when driving, does it drive straight, ride comfortibly etc. Pull the tranny dipstick and give it a sniff and check out the color of it. If you know anybody who has a DMAX take them along to look at it. LBZ's are great trucks... I absolutely love mine. Hope that helps you a bit...