I love the title of your post.
"the insurance company forced them to remove the tenants, staircase, eletrcial, etc,"...Why? What is the whole story here.
Other than that, I like it. One way to look at it is that you have already paid $100k, own 0% and have no control over what happens. At the end of your lease the rent could double or the building could be sold to somebody who wants you out, bye bye $100k. For another 92k you have complete control, your 100k investment is safer, your business/livelihood is safer, there is a huge profitability potential upstairs.
Definitely find out more about the up stairs and what can or can not be done.
If you can get a loan from the bank no problem and don't intend to finance numerous other assets then seller financing is not important for you.
Multiple exit strategy- Rent the upstairs as retail space, convert the upstairs to residential units, quit your business and rent the down stairs, sell the whole building.