Long block is about the same size as an L28, I have it's gas counterpart sitting next to one. The 300ZX trans will not bolt up to it and you'll need an adapter if you want to go that route. MBZ made some manuals that will fit: the 717.410/.411 from the cosworth, the 717.430 from the 300E and 190E 2.6 and the 6MT from the later cars which used a 2.3L M111 engine. Additionally the gearboxes from the 2.7L Diesels across the pond will fit. If you go with a 6MT in a 240Z you'll probably want a 717.663/.668 which would probably hold 450-500whp assuming you don't regularly launch the car on drag slicks, cossie dog leg a little less. Flywheel requirement will be gearbox dependent.
The Factory FI turo manis suck except for a rare non EGR 603 mani, so otherwise you'll be on your own building one. If you want to go 606 route you'll want an OM603 Turbo M pump, as those do not require the computer and have the turbo ALDA. Non turbo 606's can make 3-400 (IIRC) hp when boosted as well (smaller rods), but they also have better cams if you want to snag some for your turbo 606. Additionally all the engines are front sump, although the W140 is a sort of mid sump deal that might clear the cross member. The M104 oil pain might fit, otherwise you'll have to go fishing in europe for a pan, or make your own.