Essentially the architect didn't properly detail the separations and this was compounded by a poorly located return vent and shoddy insulation in the wall to wall area.

This is very common in the CRE world (usually using Limited Partnerships), and Single Asset Entities or Single Purpose Entities (you'll see SAE or SPE thrown around a lot) are the only way to insulate your deals from each other.Depending on what type of deals you do (flips or buy/hold/rent) you may be able to have the actual dollars flow through the operating company, just creating a paper P&L for the asset level entity.Again, I'm not an attorney, but have seen this done in the CRE world, and its the rough outline I follow for my investments.

Boston metro will be one of the most insulated market in the country so you have to consider that.

What is the sound insulation like between your space and this rental?

It may be a while before he has an assistant, and that still won't insulate him from demanding clients who want to look at homes at odd hours?  

Consider the answers to the questions above before you proceed, you can put the property under contract for 30/45 days but make sure you have an attorney review your contract, we live in a litigious society, try to insulate yourself as best as you can.

Insulation, if needed, is a royal pain depending on floor layout. 

I would have been fine with doing some reasonable upgrades in the space to be redone and some minimal-impact work elsewhere but I was not about to gut the entire building to upgrade all the insulation and firewalls and add a sprinkler system which is what they wanted.   

I changed all my windows (7) to brand new thermal windows for $2500 then I did brand new luxury vinyl siding with insulation board and nice metal skirting for $7000 I also got a brand new steel roof with more insulation board for $3600.

Our windows, insulation, roofing, tankless water heaters, HVAC systems, low flow water, dual flush toilets, etc. are pretty much standards anymore.