You may well find that you can sort the squealy brakes by simply removing the pads, clean up calliper with a spray brake cleaner & smear copper slip grease onto the rear of the pads
Obviously if the pads are knackered swap em'.
Sounds to me like you've already put some time in & spent some of your 'hard earnt' to get you to where you are now. It's always difficult deciding when to call it a day. This year alone I've put probably over £500 into my 89' 240 probably about it's value 'on a good day'.
Having just gone through the MOT, you have another 11 months to decide
Why not run it & do just the 'more needed' repairs. Then chuck it in for MOT next year & see how it get's on. As you say it seems reliable.
As the engine is such a low mileage example, even if you ran it for say, another 3 years, it'll still be low mileage compared to the majority. So you'll still have people snapping your hand off for the engine I reckon
Good luck