You can use sending units/sensors instead of using OBD systems. The difference is that you just need to calibrate the extremes and limits. Going either way, you'd be able to achieve exactly the same thing - i.e. you'd be able to create virtual gauges that you can customize completely. OBD has the added benefit that you can get other readings from the car. You can use a speed sensor (off the transmission) to get the speed, and you can tap the RPM sensor too. Most of the other gauges are already analog so you can tap them as well the same as you'd do for the RPM sensor. However, if you go with OBD, you'll never have to recalibrate the speed sensor (like when you change tire/wheel size) and you can get most of your dumby lights from it too (like oil temp/pressure, service engine soon, coolant levels, etc) really easily. No need to run a million cables all over the car.
Another benefit is that if you're hooked up to the OBD system and it throws a code, you can simply display the code on one of your screens (or clear it) without hooking up a scan tool.
The con is that you need to wait a few seconds more to use OBD than if you had raw data from the sensors. But IMO, that's worth it since you could be putting your seat belt on in that time.
Either way, you can meet your goals and requirements of a skin-able gauge cluster. I had the same goals too before I started worrying more about performance than my gauges - 2 screens about 2 inches apart with turn signals and misc dumby lights between them. It can look really good if done right...
Keep us updated.