Are you planning on adding a turbo? If so the common ones are Boost, EGT, and Fuel Pressure. If you have the room oil temp is useful if you race the car.
When you say downpipe, are you referring to the headers? Downpipes are usually associated with turbos, but you say you are a lightly modified NA car...
I have similar mods, and haven't added any guages. I probably won't either unless I add a turbo, cams or some other way to increase power by a significant margin. I feel that the stock systems can handle the 10-20 hp that the intake, exhaust, pulley, etc. gave me. I have no proof of this other than my mileage is the same now, and the car runs fine