The duratec is a tough motor but the things to watch are the rear coolant line. Check it very carefully for leaks. It's a cheap part but requires removing the transmission to replace. Same with the EGR valve. They are inexpensive but lots of work to fix due to the location. The pcv valves often get neglected because again, a bitch to get to and replace so they are prone to being never changed and then cause oil leaks and or excessive carbon buildup. The transmission should be rock solid though and if it had proper fluid changes will still be in a great shape. A 2008 wont have the intake runner control system or electronic thermostat which were nightmares so you avoided that nonsense. Other than that just check the stuff you would on any other engine. I wouldn't hesitate to jump on a 2008 2.3 with 300k if it has good service records. It uses a timing chain and not a belt so the chains pretty much last forever if the oil is serviced regularly. Valve covers can leak into the spark plug wells and cause misfires but that's a 20 dollar gasket kit that can he fixed in 30 minutes. Cant really think of anything else.
Check all that, give it a good tune up with double platinum or iridium autolite plugs, spark plug wires, new fuel filter, oil change with a good synthetic high mileage 5w20, do the pcv valve, air filter, change the other fluids, trans (motorcraft mercon v) diff (75w90 synthetic) power steering (motorcraft mercon v) and flush the brakes (dot 4) and coolant system (only use motorcraft GOLD antifreeze). Seafoam the intake and a couple bottles in the gas tank with 93 octane fuel and run it like a raped ape for the first tank.
Brakes, clutch, suspension components, u-joints just give em a good inspection and replacd if necessary.
On mine I had to replace the evap purge line, could have been a fluke but it was dry rotted and crumbling at only 75k. So good idea to go over all the vacuum and evap lines. Check the orings on the oil filler cap, dipstick, and fresh air vent elbow on the valve cover. (These can all cause small vacuum leaks when they dry out but often get ignored as possible leak locations.)
Also scan it for codes and see what pops up.