I got a single owner truck at the 290ish mark. It was 100% stock, minus 2 injectors replaced at the 125kish mark.
I have just over I'm approaching 305k. On the motor it was minus'd and de-cat'd when I got it by me and all new Ford injectors. As far as reliability mods that is it, mechanically I'll drive it anywhere.
On your EGR question, my blue one leaked as well, AS did the oring on the tube. I have some heater hose that fit and leak no more.
On your coolant kit, watch the ball-c@ck fittings as both mine have a nice gentle very slow drip.
If your Oil and coolant temps are w/in 15* of each other you'll be good according to ford. I fluctuate at 6-10*, empty or pulling, winter or summer.
If you have studs, you can do heads on, people will tell you otherwise BUT if you can afford time and money to pull it and have it decked, etc, etc, heads-on/motor in is a viable option for a truck that has had no prior issues and that has been maintained. That's my plan as if my motor is coming out it will be rebuilt or the truck will be 5.9'd.
Good luck.