My husband and I had been doing research for Serbia too for quite a long time. We joined forums and discussions groups..The best site that we found very very useful was the Global Property Guide. This is the ultimate guide for expats. They have a list of expat-friendly neighborhoods in every city, they also have a guide on how much the prices of the properties located in these neighborhoods are. On top of that, they have a list of realtors catered for expats. No sign up is required.
To look for the expat-friendly neighborhoods in Belgrade, just go to their website, www dot globalpropertyguide dot com, then go to Europe, then Serbia, then click on Rental Yields. Here you will find prices of apartments in Belgrade, rental rates in Belgrade, rental yields in Belgrade, and the upscale neighborhoods in Belgrade, expat-friendly neighborhoods in Belgrade, and a whole lot more.