how to replace a heater core?

I have a 94 Jeep Cherokee Sport & I've been leaking anti-freeze but my car isn't running hot. The garage said that the whole dash would have to come off & gave me an estimate of $800. to repair it. I have replaced several hoses that were old & worn. I put cardboard under my car at night & in the morning it's still dry so I don't see a leak. Today during a short trip to the store a 10 minute drive my Jeep started overheating. During the ride home the temp went down to normal. We checked the fluids & had to add some anti-freeze in the overflow jug because when it was parked at the store it was bubbling over. My water pump was replaced 3 years ago & it doesn't seem to be losing any water from the weep hole. I'm weary of taking my Jeep to a mechanic to replace the heater core when I'm not sure that it's actually worn out & not working. My husband suggested buying some copper beaded stop leak for the radiator to see if it possible has a pin hole in it. He is thinking about replacing the thermostat as well. I feel like I'm grabbing at straws & don't really know what the problem is with my jeep.
thanks for any help you can offer with this problem!