All depends on a good testdrive, no excuse as said for the coolant to be low.
As for the engines longevity, you can get a 4.6 to 200k if you try hard enough however it is all dependant on the past maintenance, at 150k or so the top end would benefit from a rebuild,.
Cooling system can be temperamental on the V8, plastic crap radiators the same for heater matix then no its not unusual, I had to replace the rad on mine when I bought it due to the overflow pipe shearing off.
Transmission and axles last the course.
body tends not to rust same for the chassis, however some P38's can have corrosion around the outriggers that support the rear bumper.
Water ingress, make sure the carpet and headliner are dry.
I could go on..
Basically anything inbetween the front/rear bumper can give trouble.
And then there is fuel consumption, towing a horse box will get you 8mpg, as I found out today with mine..