ModCon Recommendations

Trying to do some research for a family member (non tech savvy), who needs a modulating condensing boiler setup. I'm new to the mod-con arena, so just trying to hear some advice from on the ground with installers and longevity. I've mostly dealt with cast-iron boilers.

I know the install is very important regardless of make. Aside from that, what is your current recommendation for a modern mod con boiler? It seems the stainless heat exchangers are a must. I'm hearing about Weil McClain Evergreen Pro line after they supposedly worked the bugs out (stainless fire tube heat exchanger); Lochinvar Knight XL (stainless water tube) many swear by this design on this model even though its a water tube; Rinnai; Viessman and others.

This would be connected to a large radiator system. I know the water quality is of high importance with these models and annual maintenance. Is one manufacturer/ model/ design inherently better with cast iron radiators? Easier with part sourcing when things gone wrong? Are some more sensitive to magnetite that others? The new system would have a dirt Separator w/ magnet and a balancing valve among others. I've explained that the return water temp on these mod cons needs to be less than ~130/125 to get the most efficiencies from what I've gathered and that may introduce some contradictions with cast iron radiators. The homeowner is confident his home is over-radiated, so he understands it would not be running at 180 to get the most bang for the buck…. or so it seems like a new mod-con boiler(s) would only be more efficient on the milder winter days (fall/spring) but not on the coldest day(s) of winter.

The family member has gotten quotes with both WM Evergreen Pro or Lochinvar, possibly in a cascaded setup giving some redundancy. All things equal with install, what mod-cons do you think are made the best? Last the longest? Easier for the installer/ maintenance?

Are there other models they should be considering?

Thanks for your help