First 280K is not much...my 91 has 199K KM..not miles...that equates to around 125K miles...you would be about 175K miles if your saying 280,000 Kilometers...NOT very much for a diesel at all...
Once in a while, the coil cooler will fail due to poor condition...rust from outside in...what ever...for most part though, it's remove...clean, clean, clean, replace O rings, assemble, clean block off where gasket surface is..and install fresh gaskets and then slide rebuilt cooler in bolt up, torque to speck...done!
IP and injectors for lack of better way...are like Plugs wires and distributor caps....they do where out over time and need replacement...well so do IP's and injectors....routine maintenance...These parts are wear parts and do need replacement now and then.
I'm in a similar point...except good oil cooler...I am un aware of condition of IP and injectors...I do plan on replacement sometime shortly...funds permitting.
I would suggest you keep what you have, do the compression test, buy/rent what have you to do it. Determine that it is a solid engine then proceed with the the repairs if it's a good solid engine.
Hope this helps you.