The vendor's comment about the OPV affecting brew temp is odd, makes no sense at all to me. Maybe he was thinking/talking about the pStat at that point?
When he said that the way to adjust brew pressure is with your grinder, that makes sense. There is a relationship between pressure and flow rate inherent in your pump, where it goes to 12+ bar when the flow is completely choked off. See the diagram below:
In normal brewing the pressure is around 9 bar and is determined by the flow rate as permitted by the fineness of the grind.
The OPV on most machines (I'm not familiar with the Magica) opens somewhere above 9 bar, often as high as 12 bar or so.
The reason you may want an OPV that opens at a lower pressure is if you want to pull slow ristretto shots without the brew pressure going much above 9 bar. Until you are used to the machine and think your ristrettos need the lower pressure I'd say you don't need to worry about fitting an adjustable OPV.
Link wrote:I thought brew temp was adjusted by flushing the GH to desired temp.
Yes - on an HX machine the length of your cooling flush is the usual way to control your brew temp.