I suggest that you sell your lump for whatever you can get for it. You will want proof of life with a spinning motor if you want anywhere close to $10K IMO.
I still think the absolute safest bet is a known running 3.0CS with carbs. The motor
will be nearly free, and you can put that s38 money towards the gremlins that are guaranteed to come up after getting your car on the road.
If you are hell bent on a 3.5L, don’t buy a long block. I’ll repeat. Buy a running e34 535i, or e32 735i. Swap the motor and factory he harness over. You can get a donor for $5K or less with under 175K miles. Save those s38 funds for a refresh if/when you see issues.