That is an amazing flip and I am happy for you. I would keep it if I were you.
BTW a 6spd 850i is not "very rare". They are a dime a dozen and go for sale for sub 10k all the time.
A 1997 last year production 840i 6spd is very rare indeed, and that engine is also a gem imo..
2000 Audi A8 SWB Silver/Black
1992 BMW 525iT/5 White/Blue [MPars, HIDs, cargo net]
1989 525i [scrapped]
1995 BMW 318i DASC [sold]