If the A/C was leaking it would quickly quit working and wouldn't have much of a smell. It wouldn't take long for the a/c to totally depressurize through even a relatively small leak.

In terms of the heater core ...

It's just a little radiator that the heater fan blows air through. On the Isuzu the a/c apparently gets in the way (just been reading the manual myself) so that also generally needs to be evacuated (I think some have accomplished the change out w/o evacuating the freon).

I've heard it's a minimum of 5 hours shop time, so that makes it an expensive job. I'm afraid I'm going to have to do it myself as well.

The three different heater cores I found were at rockauto.com. I've ordered several things from there, and always had good luck.