I looked at the Hercules before I did my Cummins swap... At the time, MARS had just run out of engines due to Hercules entering receivership...
Hercules:
One of the things that bothered me about the Hercules even before they admitted that they couldn't supply me with an engine was the lack of parts availability. Filters, belts, etc. can be had at any decent parts store (although you may have to order them), but other parts seemed harder to source. There are very few Hercules powered trucks/equipment in my area that I know of.
Later on, while talking to Charles at M Series Rebuild (he installed one of the first, if not the first, Hercules repowers in a M37 for the US Market) and drove a Hercules powered M37 for years as his shop truck, it now has a Cummins in it...) was that he found that the engines routinely were excessive oil users...
Cummins:
I spot these all over the place in both vehicles such as FedEX, UPS, Frito Lay, and other route vans as well as in construction, agriculture, and farm equipment in my area. I can through a rock out my office door and hit five dealers that have Cummins powered equipement on the yard and parts on the shelf. Fiters, belts, and other such parts are on the shelf at my local auto parts store. And I'm not even considering that a lot of the parts on a 4BT are identicle to those on a 6BT and thus are avaiable through the Dodge dealers since the larger engines are in Dodge trucks (also Thomas built schoolbuses and similar on Frieghtliner Chasis...). The engines, literally, are made and used wordwide.
The Cummins turns a little slower in stock trim, but you can change out the governor spring if it bothers you. It puts out a bit more torque and they don't burn a lot of oil (or at least, I can't find anyone who claims that it is something that they have experienced on a widespread basis...). It's a heavy duty engine throughout.
Others:
In a nutshel, I also looked at GM power, John Deere, and Perkins, but liked the Cummins better for reasons of costs (initial and operating), power, availabilty, etc...
From your end, a good deal on a Hercules that is in good shape might still be the way to go. I'd consider doing one if I found a good swap candidate, but since I have a bunch of Cummins powered equipment in the yard (including a M37), I also have to factor in my own pars supply (comonality) and prior swap experience and thus would likely trade the Hercules and install a Cummins if it was my truck.
As I work on my own XM708, I keep thinking about what a screamer it would be with an ISB3.9 Cummins, but I have a fresh rebuilt 230 for it and am keeping it stock. Thanks Ed
