When insulating with closed-cell Spray Foam, what ERV and HVAC resizing considerations should I bear in mind?
At this point, it's not a DIY project. You need to consult an engineer or an architect/engineer that is used to these kind of building envelopes. A sealed envelope with an ERV needs to be designed and PROPERLY sealed, and the ERV capacity needs to be determined and designed by an engineer in order to meet building code and pass inspections. 

Note: I'm in TX and familiar with building code for high efficiency projects here, and my girlfriend is a PE but not in HVAC.