Is this a Rehab deal or a cash flowing property?

You are correct that putting 3.5% down on a fourplex likely won't cash flow after one year.

Cash flow a must, appreciation a plus.

That creates a really nice price / rent ratio which is favorable for investors looking for cash flow.

If maintaining cash flow is your primary objective, you might want to explore other options.

I'm terrified of doing all this research, finding a "dream" property, only for it to fail and negative cash flow. 

There is almost no scenario where a condo would ever positive cash flow while servicing debt, and very little positive cash flow with no debt.If your budget only allows for a condo, find a better place to put your money to work.   

That looks pretty bad for your cash flow.

If you don't intend to do that right away then do the 20% and save the extra cash flow.

I think that the cash flow you are generating from these properties is outstanding.