I did this job in my 850 last weekend. I just replaced the heater core - no o-rings. In the archives here there's lots of talk about removing the hoses between the firewall and heater core, but I didn't find it necessary to replace them.

The instructions in the Haynes manual were sufficient, but it's mostly straightforward anyways. T15 and T25 Torx head drivers take care of most (if not all?) of the fasteners.

My best advice is don't underestimate the amount coolant that will come pouring out of the core when you break the seal. Have plenty of towels laid down to soak up the antifreeze. Otherwise it will pour down your center console and footwell soaking into the foam and carpet...

Ditto on the coolant smell not having anything to do with the AC evaporator. Also, like the others said, when you have the coolant drained down for the job it's a great time to change out the thermostat and coolant.

-Will
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850 / Mini