The 3.7 from the Mercedes 2004-2006 ML350. (That's the only Mercedes that used that engine in my country).

I am doing this swap at some point. The engines are relatively easy and cheap to find. You can find high mileage ones in north america for 500$ for a high mileage car or you can order low mileage ones from japan for a higher price.

In other market of the worlds, the SL350 and S350 from that same era came with the same engine.

You make around 30 more horsepower . The bolt ons of the 3.2 fit on the 3.7. I think in a NA 6 speed it's worth it if you do it yourself. You can even get it tuned to get a bit more power.

It's basically bolt on except that you to have to put the flywheel of the manual on it and change something with the fan (I don't remember but it seemed easy).