[Cookeh]

As above, go Behr. They're the OEM for the heater matrix but have changed the design to avoid the issue that causes them to fail. I cheaper out and got a PPS one - the heat output is significantly lower and my demist times have doubled as a result.

Some guides state that you can just clamp your heater hoses and not worry about draining the coolant. That failed spectacularly for me and I drained almost the entire systems worth into my footwell. I'd drain your coolant (only adds 5-10mins, as you have to bleed the system after installing the matrix anyway) and clamp the hoses.

Regarding those hoses if they're aluminium they don't need changing unless in a visible poor state. If they're the earlier steel ones then they are worth changing for the aluminium parts.

O-rings are vital, they're actually not quite a standard profile so buy Volvo not just any old o-ring, and as mentioned keep them lubed with some silicone grease before installing. If you don't lube them they'll rip and then you'll find yourself with a leak either immediately or shortly after.

After removing your leaky matrix clean out the box that it sits in, if you don't you'll have that sickly sweet coolant smell every time you touch your heaters for months - and this will induce paranoia that you didn't quite get the install right.