You have quite the selection...
For normally aspirated you can look for a early to late M30's (late 70's 3.5ltr to 90+ 3.5ltr's found in E34's and E32's).
Then there are the ///M motors...S38 from an E28 M5, S38 from any E34 M5, M88 from an E24 M6, S50 from E36 M3's.
Or if you prefer something with boost, find a late (90+ E34 5 series or E32 7 series) 3.5ltr M30 B35 motor and order a TCD turbo kit and their big brake kit (corvette calipers) and you would have an awesome street/track machine.
...this is an M30 with a TCD turbo kit.