Replace engine?

[ERT]

So, we've got a 2002 that was given to us, had it for two years, it probably has almost 300k miles. Started having trouble getting it started, replaced batteries, GPR, valve cover gaskets, etc.

Took it to local, reliable, trustworthy mechanic:
Diagnosis, 4 bad glow plugs, 2 bad injectors
He does a compression test and cylinder 8 is only testing at about 100. The truck has been running good, with just a faint wobble in the idle, we just got to where you could not hardly get it started. Once warmed up it was running pretty well.

Do we spend the money on all new glow plugs, 2 new injectors, etc. and see how far we can get with this engine, or do we go ahead and spend the money to put in a rebuilt engine?

We like the truck, it is a ranch truck, and perfect for our needs, plus we really haven't spent anything on it to date. My husband and I are undecided.
Fix it and take our chances with this engine and hope that in six months we aren't wishing we had put that money into a rebuilt engine, or replace the engine now?
I am mechanically minded, but am not an expert on the Powerstroke. I think if one cylinder is weak, there is a reason, and it may be that the others are soon to follow.

What do you guys think?

[SuperCrutyPS]

I think a lot would depend on how long you plan to keep it.

Maybe if only a couple of years until you can get something newer, maybe just replace the parts.
If longer, maybe the engine would be the best route.

Just my thoughts. If taken care of, these buggers will run a long time.