Compression numbers are not relevant unless the valves are set to factory specs. There's 20% difference measured between certain cylinders so that's not good either. 20 vs 16. Consider that whole compr test result useless until you redo it yourself, the right way. You might want to look at the compr again but I wouldn't bother. But the never adjusted valves Must be looked at, as soon as you can. 450k without a valve adjustment is a lot (280,000miles). Postpone doing top speed attempts for now :')
Having new nozzles put into an injector is not a rebuild bc it needs to be properly set too. Ask how they rebuilt it.
The insulation some of our Volvos used is made out of recycled mousepoop mixed with M&M's candy for color, and chrystallized sugar for stiffness to make sure it all breaks apart when you touch it. Suspect it all going biodegradable by now. Simply low quality...plus think of fuel vapor (diesel mist), oil, coolant whatever and engine heat constantly attacking it for decades. The good news is that the diesels literally need just a few wires. If you decide to deal with making a new harness, carefully track and register/draw length, thickness, color and routing of each line. Dave Barton sells harnesses too.
https://www.240turbo.com/volvoharnesses.html
PS. Yeah, start with the valves now. FYI, you do not need tools, at all, to CHECK the valve clearances. Only a feeler gauge kit. Cheaply found anywhere and it doesn`t have to be anything special. something like this works well, you can have a 2nd set too to combine values and measure that way. COLD ENGINE!
https://www.ebay.com/i/401459937178?...saAmE0EALw_wcB
Order a valve cover gasket too, a 1-piece rubber version from GOETZE. Best available, won`t leak.