If you spend your $100K on a $500K property... you're done, there is no cash left for the next deal.Are you working with a lender to see how much you can borrow?  

If I can obtain the property below the value of comps, would it be likely to refinance it at an amount that would give me the cash to pay back what I borrowed for the down payment?

There were a ton of new regulations in the name of "consumer protection" that exacerbated the lending issues, and made it take much longer than it should have.Lenders may not be as aggressive as they were a few months ago, but if you have a good project and a decent borrower, you can still get your deals done today. 

Buy the paper and in some instances if you cannot work something out with the borrower (which I believe should be attempted) then you can foreclose.

If you are borrowing any portion of the money (aside from the HELOC) for your rental, it will be a different story- it's not quite a straight forward to own an investment in an LLC. 

Nearly every lender I've come across would consider a co-borrower living in the house to satisfy occupancy requirements for the loan.

The bigger issue you may face is FHA underwriting requirements are very strict on what they call Property Condition Requirements, aimed at protecting the health and safety of the home purchaser/borrower. 

If you borrow money to buy a house that's going to produce income to cover those debt payments, the operating costs of owning the asset, and have money to cover capital replacements/reserves, it makes sense to leverage. 

You wouldn't need that much for a down payment, are there friends or other family members you can borrow from? 

In the past the VA borrower got cash back at closing using this 4% seller credit strategy but no more longer.