This is not as easy a DIY as it seems. First, you have to determine the appropriate method of protection (Line 1 to Neutral, L2 to N, L1 to G, L2 to G or L1 to L2). Then you have to know the available short circuit (symmetrical) current available (most devices are limited to 25,000 amps) and there has to be a breaker space available for the device itself UL listed for one or two wires per terminal.
I'm not sure I understand your question: do you want the entire service protected or just the 50-amp dedicated (sub?)panel? If you want the service entrance protected, there is no need to protect the subpanel. If you want just the subpanel protected then this panel has to have an equipment ground bar with a grounding conductor back to your service panel.
I would retain a consulting engineer to anaylize what you have and to give you something to take to an electrician.
As far as whole house models available, I prefer the Square_D "SurgeBreaker Plus" model SDSB1175C.
I'm not sure I understand your question: do you want the entire service protected or just the 50-amp dedicated (sub?)panel? If you want the service entrance protected, there is no need to protect the subpanel. If you want just the subpanel protected then this panel has to have an equipment ground bar with a grounding conductor back to your service panel.
I would retain a consulting engineer to anaylize what you have and to give you something to take to an electrician.
As far as whole house models available, I prefer the Square_D "SurgeBreaker Plus" model SDSB1175C.