I'd take that financing quick if you can, saves you the hassle of using a bank. 

The situation with me is that I have about $150K that I have saved up, but this property that I REALLY like is listed around $320K and isn't financeable.

Continue learning and continue saving. 

We deposit to a savings/checking account set up for the rentals, to more easily track rental income and expenses. 

Easier to afford retirement and if they are selling a business, the taxes along may pay for their house. 

When I first started doing lending in Detroit.. this was an issue ... my client a big time flipper rehabber always had retired cops there with a clip board to check in the workers.. it was determined early on that much of the theft came from the exact guys that put the stuff in during the day.. they had the codes they knew what was there they would come in at night and steal it .the day cop and clip board put that to bed.So I agree with @Bryan Bannach  hire a retired policemen have him there during construction hours its the cost of owing property in the hood.. nothing unique about this.or just sell the property and buy something else.. but I think your looking to assemble so I know you probably don't want to sell.so just as stated hire security and I really like the idea of the @Patrick Liska  Cinder block fence I would for sure do that.. then put broken bottles in the concrete on top..

They are semi retired but probably have about 5-10 years of income left.

I also have roughly $30k in savings that I could use for my down payment with another $10k in reserves that I'm not willing to deplete.

If you're going through a realtor to purchase it that realtor should be giving you a discounted listing on the resale so that should save you a % or so.If you want to PM me I can comp it for you if you want another set of eyes on it

My primary objective is acquiring decent cash flow and the savings that I have aren’t tied to any kind of debt.