Any ideas on effectively treating the bedbugs and hopefully minimizing costs?

I’d keep it very basic with pressure treated framing, plywood door, and mesh chicken wire between the units.  

That is how they will treat your home.

It's important to remember that 4-plex valuations are treated the same as single family homes...that is based on comps. 

Wait until the property legally belongs to you before treating it as yours.

I would move those hoarders out ASAP, no one would ever treat one of my units like that.It looks like an auction, what makes you think you can buy it for 260K?

If you submit an offer with a high nonrefundable EMD and close fast it will be treated as a cash offer.

Each Partner could contribute a different percentage of the total and could be treated differently. 

If I looked into the BRRRR method I could treat the money as a private lender, just with slightly lower interest rates to still make it a good personal deal but still get a small profit with someone I trust (myself).

You can remain 50/50 and the additional money treated as a loan.