Was the engine good and the issue was just bad maintenance or was the design bad?
I think the engine is just fine if you use it within it's design limits. The biggest issue is the cost of rebuilding an engine. Parts are getting harder and harder to find, and some people want a mint for parts. I agree with Beast a lot of these trucks were abused or neglected and the engines suffered the worst of it all.
2nd question:
What is the 'best' engine?
What do you want it to do? People have put in fire breathing 454s, multiple diesels makes (6.2, 6.5, 4&6 bolts, Ford diesels), Lucky put in a fuel injected Ford 300, etc. Is this a putzing around truck or something else? I totally agree with Kaiser715. Chevy 350 with your choice of transmission is a great simple combo. You have to decide how exotic or original you want to go.
3rd:
Site recommendations to possibly buy my own M715?
You can find a craigslist searcher that does the entire US. I just did a google search of "M715 for sale" and had a large number of sites to check out. Some of the prices were pretty dang shocking, though! You can also put something in the Wanted ads below.
This site is by far the best informed group of M715 owners. And a very polite group of gentlemen as well. There have been quite a few variations of the Zone that changed due to updates or server crashes that you can search in the archives or the PseudoZone archives. They're sub-forums under the bigger general tech lists. Lost of great information to be searched there as well. Good luck on your search, and certainly feel free to ask any technical questions about the truck for your book- gonz
Come and take it
Go work at joann fabrics if you can't shoot a gun