Reps need to know the basics... Who their customers are, how to get there and enough in information about your products to sell or merchandise them effectively... There is not much argument about that... Where the opinions really start to diverge is on the topic of how much additional information can reps handle before it becomes more of a burden than it is worth.
There have been thousands of mobile solutions built over the years to extend corporate data 'the last mile' out to the rep when they are in the field. Early 'handheld' technologies ran on tiny amounts of memory by today's standards, and typically left reps starving for more details... Then laptops emerged to give us nearly unlimited data distributed to the field, but studies showed that most of these 'mobile' applications left 90% of the data in-used and for the most part in the way. Only the most advanced users got any real value out of having deep details about customers and products available to them.
The good news is that mobile technologies have continued to evolve, and along with them, we (application designers) have gotten much better at helping users navigate larger amounts of data. Touch screens that leverage highly responsive high-definition displays make it easy to build intuitive interfaces that field reps can use to quickly and intuitively access client lists, contact information, maps & directions, purchase history, visit notes, product details and much much more. The real answer to how much data a rep needs is finally based on the business case rather than the limitations of the technology.
As long as the interface for accessing the data is quick, easy to use and intuitive, reps won't be bogged down or distracted by superfluous data elements.The best practice is to map out all of the possible scenarios that a rep will face, and identifying all of the data that is required to efficiently get through those scenarios. Organize the data so that the minimum data required to get the task completed 80% of the time is automatically made available when the rep starts the task, and that the details required to complete the 'fringe cases' is available via straight forward and intuitive navigation.
This approach seems simple on the surface, but a whole branch of user interface design focused on information architecture has evolved to address the issue. The more complex the data you are dealing with, the more attention needs to be paid to how the screens and workflow are laid out. It is critical to test the interface with both 'the masses' who may not be considered power users, as well as your most advanced users. You will need to achieve a balance between making the solution intuitive enough to deploy with little or no training, and not slowing down your advanced users with prompts and directions.
If you can achieve this, or better yet find a solution that is already architected with these principles, the right amount of data to provide to your reps should only be limited by the questions they might ask while working in the field.