There are lots of keys depending on you strategy and how much work you are willing to put into the property.
What is your investment strategy? Appreciation or Income. If you picked appreciation, you're probably buying a plex, condo or single famly home. These appreciate with the market and use conventional financing. If you picked income, you need to run the numbers to figure out what you probable income is and you are probably looking at properties bigger than 4 units.
Maturity - When does the property mature (i.e. How long before it is profitable?) Granted...I'm in California, where finding a mature property from the start is a difficult proposition. The general rule is that you should get to maturity in 18 months based on your initial equity level. If you're doing a 1031 exchange you should reach maturity immediately.
As a general rule, plan on buying for the long term. This will take market conditions out of the equation. At least in California over the long term the property will appreciate and the rents will go up. The longer you hold it the more money you make.
This advise is California specific. If you are looking in other areas in the nation, you should be looking for mature properties from the start.
here are two basic rules to follow which should keep you out of trouble...
1. Never pay market for the property...find the deal.
2. You're not going to live in it. It's an investment. Don't buy it because it's pretty. Buy it because it makes money.