Can't Seem To Purchase My First Property.

We were contemplating a property and checked out its numbers recently. The Annual Gross Rental Income=73200. and with a 25% vacancy rate (common here) the Effective Gross Income=54900. . Annual Operating Expenses are : Taxes=13,860. Insurance=1,800. H2O=700 Garbage=360 Electric=2,750. Landscape (grass, snow)=800 Management @ 6%=4,392. Budget for Maintenance/Replacements @10%=7,000 TOTAL OPER EXP=31,662. This leaves me with 23,238 NOI… 1936.50 a month…. BEFORE DEBT SERVICE. Asking price is 660,000, but even if we (by a miracle) purchased it for 500,000 I don’t see how I could afford a mortgage with only that. These are only reflective of 11months (except the insurance), so the end would even be less NOI. I have three questions. First, what is the opinion of these numbers and my calculations? Second, should I just skip this and instead of putting all my “eggs in this one basket” go purchase a few four-unit buildings? Lastly, in my calculating I realized how dependent I am on those “on-line” mortgage calculators. You know, the ones that tell you how much you’ll pay monthly in for P/I based on the term length, percentage and amount borrowed. Well, I was told that it is better for me, and the only option given by the majority of lenders, to use a 5/1 ARM. When doing my cash flow calculations, I was stumped how to figure out the monthly payment for a 5/1 ARM. I couldn’t find any calculators on line that I understood. Can someone tell me how to figure this out? 8-/ thanks!