Now, as per our contract, we were to pay 30% up-front, 40% after demo, 20% at 75% mark, and 10% at the end).

However I've helped my parents and friends do different home demo and renovation work over the years. 

In this situation, I would imagine that I have to demo the back units, kick out the tenants, and re-do it from scratch?

We'll probably go ahead with the purchase, realizing the extra testing and possibly precaution that will be necessary if and when we drill, demo, rip out flooring, etc. 

Do I invest 300k into a turn key duplex in Worchester that cashflows $800 a month as my first investment or do I try and find a BRRR property with no experience in demo's/construction where I can recoup my down payment and do it again. 

As in if it's just a few treads demo and repour those few treads.

The less stuff you need to demo/replace the less cost. 

For example looking for a 2-1 and adding an extra room, and/or some demo work to create an open floor plan for kitchen/living.

Started demo in a condo and ran into these metal brackets under the carpet at the front end of the unit.

Create a demo real, specific to MF, and post on LinkedIn, FB, and Youtube, etc.