IMO $18,800 is a real premium price for an 11 year old truck. The new engine is potentially a nice bonus (more on that in a minute), but the rest of the truck still has how ever many miles are on the odometer, which is?
As for the engine, where did it come from? Is it a Ford reman? Jasper? Ashville? Used from God knows where? How much of the original engine's supporting parts did they reuse?
There's a lot to look for when buying one of these trucks, but the first thing you need to think about is do you really need it? Are you going to be towing or hauling heavy weight? Because if you're not I would respectfully advise you to consider buying a similar truck with a gas engine. Diesels are great if you need them, but there's a cost to be paid for that ability. Diesels are more expensive to maintain, and require more frequent maintenance. Parts are more expensive (how does $1,600 for a set of injectors grab you?). The 6.0 is a good engine, but it does have some trouble spots. These can be corrected with the installation of the proper parts, but if the truck needs to be fully bulletproofed consider that you will have another $5,500 you will need to sink into it (assuming you are paying for labor) at a minimum.
A 6.0 purchase is not one to be taken lightly. These engine are complicated and can be troublesome if they were not maintained. I would do considerable research before buying any 6.0 truck because you need to know not just what to look for, but what you may be in for. I don't consider these engines to be the problem a lot of people on the net would have you believe, but I've also owned mine since new, so I know how it was taken care of. When buying used you are always potentially buying someone else's problems.
'06 F250 4x4 - 5" Flo Pro exhaust, SB Filter intake, Accufab elbow, Edge Evolution (monitoring only), SCT w/ ID custom tune, FASS 195 pump, Gillette Diesel EGR cooler delete, Sinister Diesel coolant filter, ELC coolant, updated turbo drain tube/oil feed line/STC fitting/oil cooler, ARPs w/OEM HGs, Elite coolant lines, ITP RR fuel system, RCD 175/30 injectors, Powermax, BD CCV, FICM.com FICM w/ ID tune, Elite UP, BPD water pump - 13.069 @ 101.94
'02 WRX - Outback rear disc swap, EBC green pads, DBA pillar vane rotors, TXS UP/TBE/TMIC, Perrin LW crank pulley, PPG billet steel shift forks, ACT Streetlite flywheel & clutch, K&N filter, STi Group N motor/trans mounts, TiC/Kartboy rear diff mounts/subframe lock bolts/outrigger stiffeners, Kartboy SS & all shifter bushings, custom PDX tune for Cobb AP - went 14.1 on a terrible 60ft before most of these mods; shooting for 13.50s