I really like my Isspro gauges, they seem to be build very well. Just be sure to get one with a long enough capilary cord, Seems like the 6 feet ones are pretty standard and you can't reach very far with that.
take the sending unit wire and ground it out to a good ground and turn the key on. If the the guage goes all the way up the guage is good, if it does not the sending unit is no good or you have a broken wire or bad connection some where
actually you could run an aftermarket elec gauge which will not have any coolant concerns but the old 4* series gauges were much better than the 60 70 stuff.
i suck at elec so i have no suggestions for you,
its basically, positive from the battery through the fuse box to the guage, which is grounded through the sending unit to the engine. As the sending unit heats up, it decreases in resistance, increasing the voltage at the guage.
Like Waaz said, the wire that is connected to the sending unit on the thermostat housing, unplug it with the ignition in the ON position and have someone hold it against a ground, like the thermostat housing. If the guage goes up, the sender is fried, or you just have a bad connection. Otherwise you will have to trace the power to the guage from the fuse box.