...I've replaced all the brake calipers and rotors, the blower motor for the heater, and installed a new engine wiring harness...

You've lived through the worst parts of owning a 240, and if you can still say that you would consider owning another one, I'd say do it.

You could go all the up to end of the production run (1993), and you'd be pleased at how little they changed over the years.

You'd probably be OK with a 740/940 also. The body is a bit different, but it is still quite roomy and uses virtually the same drive train as your 240. Your biggest disappointment with the 740 would be the durability / build quality of the interior - nothing short of a 1960 F-100 Ford pickup beats a 240 in that department.

Stick with a B230 8 valve engine. If you start getting into the 16 valves or 6 cylinder aluminum engines, or front wheel drive, you'll be venturing into unfamiliar waters, and from what I read between the lines of your post, you don't need that stuff.

Of course this is all just my opinion.