I'm still tuggin on this thread.....(dog with a bone)

So Wiki lists the M112 E37 engine (3.7L) as applicable to.....
2003–2005 ML350
2003–2006 S350
2003-2006 SL350

So I had a dig around and it would appear the 3.7L version of the ML350 sits mainly in the W163 models and is the most common and hence the easiest and (I presume) the cheapest to source.
Low km would be the key here, unless your going for a full strip down.

I found this with 77k mile engine for example,
Mercedes Benz ML350 W163 2004 Engine Tested Motor | eBay

just up the road (about 2hrs) from me as well.....
Would still need to do a fnial check on all the mounting and tranny bolts to see if they are identical, sometimes the slightest variants can cause a major PITA.
A knowledge of engine numbers would be good - I did a quick google without success.....anyone?

Idea being - pull out the 3.2L AMG core motor and put this 3.7L in and re-attach all our peripherals + charger + maybe a 178mm crank to help the charger make up for the added capacity.
Run it 'till it complains and then rebuild it with M113 AMG parts.

PS - you possibly even diligently part out some of the intake manifold etc and recoup some of your outlay.