I hope this isn't too general of a question; I could really use some seasoned advice.

I am newly employed as the sole "SW Engineer" in a fairly small shop of scientists who have spent the last 10-20 years cobbling together a vast code base. (It was written in a virtually obsolete language: G2 -- think Pascal with graphics). The program itself is a physical model of a complex chemical processing plant; the team that wrote it have incredibly deep domain knowledge but little or no formal training in programming fundamentals. They've recently learned some hard lessons about the consequences of non-existant configuration management. Their maintenance efforts are also greatly hampered by the vast accumulation of undocumented "sludge" in the code itself. I will spare you the "politics" of the situation (there's always politics!), but suffice to say, there is not a consensus of opinion about what is needed for the path ahead.

They have asked me to begin presenting to the team some of the principles of modern software development. They want me to introduce some of the industry-standard practices and strategies regarding coding conventions, lifecycle management, high-level design patterns, and source control. Frankly, it's a fairly daunting task and I'm not sure where to begin.

Initially, I'm inclined to tutor them in some of the central concepts of The Pragmatic Programmer, or Fowler's Refactoring ("Code Smells", etc). I also hope to introduce a number of Agile methodologies. But ultimately, to be effective, I think I'm going to need to hone in on 5-7 core fundamentals; in other words, what are the most important principles or practices that they can realistically start implementing that will give them the most "bang for the buck".

So that's my question: What would you include in your list of the most effective strategies to help straighten out the spaghetti (and prevent it in the future)?