If you are staying stock the fuel pressure is the only one you really need IMHO. The boost and EGT are nice to have though. The pillar mounts are pretty easy to install really, and the gauge connections a piece of cake. If you can do a stereo you can do gauges with your eyes closed.
I guess if I had the money for a road relay I would do that in a second, it will tell you things the guages can only dream of, it is not even in the same league. If you can spring for it I would but you still need the FP as the Cummins has no idea what the FP is. The road relay cannot tell you what the EGT is either, but again if you are stock it really is not a concern. Hope this helps.