You have a 50-50 chance of buying someone else's financial disaster area with any truck newer than model year 2004. Emissions trucks are a total gamble.
I'd get on the Truckpaper website and look for the specs you want. Call the dealer and order a full service oil sample lab if they haven't already changed the oil... it'll tell you a lot about the truck if you can get to it soon enough. Order a Rig Dig report, run the VIN number, look to see if its been put out of service frequently (bad in-service maintenance,) and reject it if it has. Have the dealer run the ECM report, and look for anything suspicious. Also run the VIN number past the manufacturer to see if its been in their shops for anything serious. None of this should cost you more than $100 for any truck you're serious about.
If its still passing the smell test, NOW its time to take it to an independent mechanic for an inspection, dyno it to check power to the ground and crankcase blow-by. Along with everything else, ask about the condition of the front end suspension. It will tell you a lot about the how the truck has been cared for. You want at least 80% of the rated horsepower delivered to the wheels on the dyno, and blow-by will tell you something about how near the need for an in-frame is. This will also tell you what it will take to make it road-ready.
You're not looking for reasons to buy it... you're looking for reasons to reject it. Make the truck convince you its WORTH being bought. If a dealer doesn't want to provide you with the opportunity to do any of the above, walk away: they're hiding something. Don't buy a truck that will financially ruin you.
There's great trucks out there for about $10,000 that will do you well with less than the same amount invested in it to make it road-ready that will make you a ton of money.