Run your #'s and so long as it makes sense and hits your objectives, I'd go for it, even if you think that it sounds expensive.

An appraiser's job is to give an objective, impartial, and unbiased opinion of market value.

@Kristin Cooper you don't say what you're objective is but if it's cash flow, this is a little hard to get excited about with a 4% CoC return.

What is your long term objective for this?  

So I’d look into what it will take to make the basement unit safe, get some bids, and go from there (possibly negotiate an inspection objection from the seller for the cost to remedy the clear safety and health issue).   

My objectives are 8 to 12 year payback. 

The other two stand alone buildings are separate individual owners.My objective is to do a JV split agreement similar to the way you mentioned. 

However, if your objective is to get out of rental then its best to package your portfolio and cash out.

No system is perfect, so don't waste your time chasing after the next shiny object.

Use your education/knowledge to help decide.Define your goal and your path to get there becomes less cluttered with shiny objects that can distract you.