Saul,
I have a 91 940 turbo with 200k and any starting problems have been related to what you have checked. You indicate that you have fuel, however, did you check for spark? Pull the coil wire out slightly and have someone crank or pull a plug wire and attach an extra plug to it, ground the outside and see if you get about 1/4" spark jump to ground.

A more rare thing would be the coolant temperature sensor, but with 300k ...

If your injectors are not buzzing, then how about your power stage unit mounted on the drives side of the fender wall just aft the battery? I just bought one yesterday for $80 for my 86 740 with 200k.

I'm no expert, but have been there with an intermitent no start, had several mechanics attempt to fix and could not, and I found myself to be radio suppression relay. To double check, swap with radiator fan relay (same relay).

One more thing. Is it could were you are? I have had a starting problem when weather turned very cold, and turned out to be spark plugs. I did not get any firing of cylinders, just turned over.

If you have never replaced your fuel or radio suppression relay. It may be a good time to re-solder the boards or buy new relays.