1995 Nissan Quest Temp Guage

It has been cold here (Chicago) and lately my temp guage seems to just stay at cold, even after driving for 1/2 hour. The heat is working well. Also, there have also been small puddles of coolant on my garage floor in the morning but for some reason not when I leave the car parked outside at work all day. I was told about a year ago that I had a small leak in the radiator and I used some Barr's Leak stuff to try to fix it temporarily. This led the mechanic at my dealer to tell me last month that I had oil in the coolant which I don't think is the case. (I think it was just the Barr's Leak which is brownish and I did not tell them I had used it.) Anyway, is the leaking coolant connected to the temp guage problem? I check the coolant level every week to make sure I have enough. How much would it generally cost for a new and/or rebuilt radiator? And if you think the temp guage problem is due to a thermostat or sensor or something, how much to repair those items?