Not a bad price, but they can go a bit lower i think. As far as issues. First: ask for the service report for that vehicle, if there were no issues with it, it should be ok despite the low mileage. Second: it was a demo soooo chances are it was not treated well nor was it probably broken in correctly, so keep that in mind too. You can get that same truck brand new for 46,000 depending on your negotiating skills and how badly you want the truck
If you do decide to go with the 6.4, then most of us here will tell you that its a great truck, hands down beating the competition in fit and finish and motor strength. Things you should do to protect your large investment would be to first: install a fuel/water separator kit i.e. Air dog $500. Second: Buy a Spartan, about $1500.. I didnt buy mine for the extra 275 hp 450lbs torque, but its nice
I bought it to stop the regen cycle from happening, which all new diesels do by the way. This is a EPA thing that is killing our MPG and all other "new" diesels mileage and perhaps the longevity of this AMAZING engine, by having excess fuel enter the oil system. From my research Spartan is the only tuner that isnt easily found by the ford system; however there are some savy techs out there so keep that in mind. By no means am I saying you MUST tune your 6.4 (it CAN void a good portion of your warranty if its tuned and brought into ford services for warranty work and it was the tuners fault... you play you gotta pay). You just have to keep up on your monthly maintenance schedules and keep a sharp eye on your oil levels.
I believe you said you are new to diesels. If that is the case you must do your research. There are allot of extra things you NEED TO KNOW that differs from gassers. This site has a wealth of knowledge and some damn good techs that are willing to answer questions.
I hope this helps in your decision making process. Honestly I would see about a NEW 2010 first before i'd get a demo. But if the price is right, the warranty is right and there were no major issues in that vehicles history, then go for it.
BTW... I love my 6.4.. its truly a awesome all around vehicle. It just needs some minor tweaks to make it perfect