If you want your data system to help you be a better driver, then you want to focus on the sensors that pick up driver inputs. Those will include steering, throttle position, brake pressure, lateral G, Long G, and engine RPM. A very nice extra is to have a GPS antenna over a lap beacon as it will give you the driven line and also work automatically, versus having to put trackside beacon every session. Plus, invariably, you will forget the trackside beacon at the track and have to purchase a replacement (or three).
I find it very helpful to lay out a diagram of what sensors you will have, how many junctions you will have, and the rough position of the sensors. This allows you to plan for how much cabling you will need, where to put junctions, what will hook into the system, and give you a good overall idea of the project ahead of you.
Once this is done, it’s a good idea to double check your plan and make sure that you are configured to match the specifications on your dash. Many systems have limits on what certain channels can do. For instance, in an AiM MXL, wheel speeds must go to a specific channel to go straight into the dash, but can go into a Channel Expansion if the Channels 1 and 2 of the expansion are both set to speed. Some systems also place sampling rate limits on certain channels, so it’s important to plan sensors that will need a high frequency (like shock sensors) to go on the channels that have the highest sampling rates.
This planning, which can be a pretty involved project, will assure that you get the data system you want. It will also help you to hone in on what you want out of your system and make sure you that you have the right sensors and configuration to get the data you need.