I replaced the heater core outlets on my 1998 C34 Stagea Series 2. This guide was absolutely spot on. Apart from getting the evaporator out this is a pretty easy job for you to do by yourself at home. I reckon it took me 3 to 4 hours tops.
I got replacement heater core outlets from Boostdoc: https://www.boostdoc.com.au/. Very well made and highly recommend them. Note they are angled slightly differently to the factory plastic outlets but they fit and getting zero leaks - Aaron @ Boostdoc says that they are angled that way so the hoses sit flatter against the firewall inside the engine bay and give more clearance behind the head.
Only additions that I would have to this guide / little things couldn't seem to find solid info on or missed in my homework:
1) BEFORE you start, get yourself some new cabin filters if you haven't changed them recently - mine looked like they had been there since '98... Repco RCF708 fits, equivalent of Ryco RCA318P. Also give the evaporator unit a clean with a compressor and/or vacuum cleaner while it's out of the car.
2) With hindsight removing the passenger seat probably would have made this a more comfortable job to do, especially when you're removing the heater core outlets or trying to remove the plastic tab holding the core in.
3) The heater core itself is fully alloy.
Got some pics of it installed as well as that plastic clip that was mentioned in the guide. I tried to be gentle with it and it snapped. The hooked end in the pic was closest to the firewall. Not sure if it's hinged or like that on the other side as well. I'm not worried about the core going anywhere.