Master Cyl lived a respectable long life, just keep an eye on it. Problem if it leaks too much it can spill into the brake booster. I don't think that is a crazy expensive repair. Maybe 1.5 hr labor? 75-100 part?
That PCM issue wont help bad 02 sensors. It just makes it much less sensitive for emissions reasons.. It may help if you keep it long term though. The 02 sensors have to work optimally, for lots of systems on the car. I wouldn't worry about the crankshaft seal unless it is gushing oil. Keep an eye on it. I've noticed mine comes and goes. I also have a tiny front axle pinion leak, maybe add less than 1/8L at each oil change. Not worth to bother. My neighbor bought a new explorer and it has tons of problems. Some he has to just live with. They changed a noisy hub and it looked worse rusty than a 25 yo one.
Very few cars can be driven reliably 25yrs down the line. These repairs(not counting incompetence) are relatively minor.
I will for sure have to keep an eye on it, that and the crankseal leak and the red silicone on the oil pan. I didn't think it was that expensive either to replace the master cylinder. The quote he gave me was with him bleeding it after and if the combusion valve has to be changed too. It was a rough quote since he doesn't know exactly how much the parts are at the local shop. He quoted me 2 hours labour and up to 4 if he runs into issues. 1.5 hours to replace everything and 0.5 hours to bleed including doing a full brake bleed on the whole system. He has a tank for it so he can rebleed any car in 30mins.
With the truck running, there's no oil that comes down to the ground and if there is, it's one drop in many many minutes. Before I had red silicon applied to the oil pan cracks, there was 2-3 drops every couple of seconds so it was much worse and consistent. But in the morning after the truck sits overnight, I get about 5-7 drops of oil on the ground. The new high mileage oil could be a factor. Will give it a couple of oil change intervals to see if it helps slow the leak. If not then I will just go back to conventional oil or the next oil change I may try to find 5W-40 (tough to find in stores here).
I need to give it more time but so far I'm not losing that much oil compared to before all these repairs. I don't mind topping up a little bit every week if needed.
What year explorer for your neighbour? Brand new?