Just so I'm clear we're gonna spend more money than it takes to fix a problem on putting in a different engine with less power, that has it's own problems. Make sense. I guess
If you were gonna do that, go with the S62 reliable and more torque or pony up to an S63 or be a real man and move up to the the S70/2
Big difference is the S65 is much closer to plug and play electrically and mechanically. Like there might be a little bit of tweaking to do Re: EWS, but nothing major. For everything else, you'd basically have to emulate the E6x/E9x DME messages. Of the engines you mentioned, S63 would probably be the most doable, but still would require far more custom work.
Stroker S65 can be interesting since those also fix the rod bearing issues... but you're looking at a huge cost at that point, even without taking into account any custom work you'd need.
On the topic of spending big money to fix deficiencies of the platform, I do wonder if it's feasible at all to modify the S85 heads to accept the low pressure VANOS.