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Congratulations! This is an exciting endeavor.
Learn about CAP rates and other jargon so you can communicate real estate investment like a pro. Consider this book HOLD by Gary Keller for full blown training.
My advice is to ask the lender for your limitations. How much down payment are you investing? You will need $$ leftover for repairs.
How much debt to income ratio will they lender accept for you. It's going to vary between 2, 3, 4 units.
Learn how much appliances, materials, labor, etc cost for remodeling and repairs.
Figure out your comfort level of buying into appreciation-heavy or cash-flow heavy neighborhoods. They are very different. Everybody says they want the middle, but that's elusive and weird.
Please go to www.biggerpockets.com and research, research, research. Listen to the podcasts there is a ton of information. The duplex will not "profit" $500 per month if you haven't even included maintenance yet.
I'd also read http://www.amazon.com/HOLD-Find-Rent-Houses-Wealth/dp/0071797041 that book.
Best book I've read on real estate ever. Follow this advice and read the book, you'll be well on your path to success. This $32 book in itself has made me hundreds of thousands of dollars.
Best book I read that is pretty much all you need to know to get started for buy and hold: