Cheap Fix for Heater Core and 3000GT

I am looking at Diamante's and got the typical smell from a leak in the heater core. Seems I read somewhere that all that needed replacing was the 0 rings and not the entire core. Of course there is the labor involved. I wonder if someone reasonably handy could replace the O rings themselves or is it highly reccommended that a professional do it. Considering I have read that the job can cost upwards of $900.00, I would like to find a less expensive fix.

Also, I was shocked at the amount of power these cars have. I own a 98 3000GT SL and it sure feels like the Diamante would wipe the floor with it. The 3000GT, in those I have driven, have puny low end torque and doesn't start making real power until about 4000 rpm's, alright for a Manual transmission but a disappointment with an automatic. The Diamante feels like it has good low in torque with a nice linear power band. Comments??