Oil pressure is always good, and should be at the top of the list. A proper coolant temp guage (as opposed to the "dummy" guage on the dash) is also a very good choice. Oil temp is another good choice, especially if you live in a hot climate. Trans temp is usually only necessary if you have an auto trans and do a lot of towing, or other high load low speed driving, and I don't think many people with contours fit this bill.
Voltage is a nice one to have, but not usually in my top three on most cars. Manifold pressure is nearly a necessity on a boosted car, but on an NA car it doesn't do much, other than give you a rough guess at your instantaneous fuel economy. Cylinder head temperature is pointless. I'm assuming you are talking about pulling it from the OBDII port feed, which means its not actually measured, but inferred based on a table built into the processor. Don't even bother with it, it is only in there to help Ford calibrators while they are working on the original tuning.
On the Scanguage II, never used that one, but I have monkeyed around with the original scanguage unit in one of our old race cars, and we found that the MPG figures it records actually are fairly accurate, even when on the race track.
