That's a pretty tight margin deal so make sure you have the numbers nailed down.

Take a piece of scrap 1/2 plywood and some construction adhesive, applying it over the crack and nailing with 8 penny nails.

Generally mold/mildew/fungus will only grow in a wet location without light and poor air circulation.

I've even moved all hist stuff out of the area, and barricaded that unused entry way (plywood and nails), and posted no trespassing signs, only to have him tear it all down and move back in before the next morning.My biz partner gave the police an earful, and they said they'd patrol the area more frequently (stop by whenever they're around), but I'm not too confident that anything will be done.So, I'm brainstorming for some options.

I used to leave a combination of small flashlights, Cadbury Bars, single serving bottles of wine, nail clippers or a multitool on every bed. 

Those 15 minutes tell you if you want to take a close look at the property, where you'd really try to nail down numbers as close as possible.Here's how I do ARVs. 

I would nail that down before making the decision on where to invest.

@Alan Rohrer pretty much nailed it. 

Nail down the ARV, multiply by .7 to .75, subtract your estimate for rehab (since your new, add a 25% - 50% buffer), then subtract your fee.

To the man who only has a hammer, everything looks like a nail.