Is it a Ford dealer? Ask them for the Oasis report. If not go to a Ford dealer with the VIN and pull the Oasis report. That'll give you a good feel for problems / recalls / warranty work the truck has had previously.
Not knowing where you're from, given the stats you've said regarding the 04 250 4x4 with 149k, assuming it's not an FX4, that truck would go for around 16 - 18k around here mainly due to the mileage.
Before starting the truck, while the engine is cold, open the coolant cap on the degass (overflow) bottle and look for oil floating on the surface of the coolant, that could signify a head gasket problem. White smoke could signal either head gasket or EGR cooler problems.
Go to Autozone / Oreilly's / etc and pick up a Scangauge II ($200). While driving the truck on level ground at 65 mph the Engine Coolant Temp (ECT) and Engine Oil Temp (EOT) should read less than a 15* difference between the two (with the oil typically being higher). Only take this reading after the ECT has reached 190*.
I believe the Scangauge can also read FICM, if so the FICM Volts should read 48v at Key On Engine Off, while cranking and at idle. This can vary slightly but should never drop below 46 Volts. If it is then you'll need a new FICM module. I'm not 100% on whether the FICM Voltage can be read with the Scangauge tho.
The Scangauge can also read any engine codes you may have which can also help you make a descision of the health of the truck.
Overall the 6.0 is a great motor and most, if not all, of the problems are well known and easily fixed / bulletproofed / repaired all depending on what YOU are going to want to do with it and use it for.
Whichever route you decide, good luck and welcome to the org!!
There's a wealth of information on these forums regarding the 6.0 and reading this post will be a good starting off point for you.
Things to consider about owning a 6.0L