My gut is saying something different.If you're looking at converting your Primary home to a rental, then you're (almost) never going to find a better refi rate than what is available now.  4.375% is a great rate - absolutely nothing wrong with that, however how low a rate could you get, if you asked around? 

So I'm in the process of purchasing a single family in Massachusetts and would love to get any input on the topic of converting a single family into a 2 family.

I converted an old Rollover IRA to an SDIRA to do my first investment in a syndication.

Rezoning allows for more units, either through converting a SFR into a -plex and/or add ADU or DADU on the land. 

It is attached to the rest of the house, but it is a converted garage.

I also knew that my house was bigger (had an attic that could be converted into a bedroom) and had a bigger yard. 

Probably so much that you'll be overwhelmed , but please remember that there's: Acquisitions (buying of Existing Self Storage Facilities); Development & Conversions (building a facility starting with dirt or converting an industrial warehouse or Big-Box Retail store into Self Storage);  Raising Capital; investing in a Syndication, & so forth.And there are plenty of folks here on Biggerpockets who have started forum topics, added posts, and greatly contributed by sharing their knowledge & experience. 

After the previous owners’ children moved out, they converted one of the bedrooms into a dining room.

I'm wondering if anyone has experience in northeast NJ (Bergen, Essex, Morris, Passaic County) with buying a SFH and converting it to a two or three family.

Try Knightsbridge foreign exchange , if you need to convert cad to usd and send it to a US based bank account .