Repair or replace engine

I've got a 2001 F250, 7.3 Powerstroke, with 300,000 + miles on it. Have had an oil leak for a number of years that has gotten a lot worse lately. Was preparing to take it in to have that looked at but on Monday, I looked under the hood and noticed there is diesel in the coolant reservoir. Through a little research, sounds like injector cups probably need to be replaced with small chance of heads being cracked. Injectors and glow plugs have never been replaced so thinking I should have them replaced also. The injectors have been sticky for a number of years. Also, thinking my oil leak may be rear main seal, but haven't verified.
My question is, should I replace cups, injectors and GP's for about $4500, replace all the above plus new heads for about $5500, or replace whole engine for about $12,000? Truck ran great, I would like to keep it.....I know nobody can make decision but myself, but would like to hear opinions from more experienced 7.3 drivers.
And I do understand the damage the diesel will do to hoses, etc....that will be taken care of also.
Appreciate any input. Thanks!