I am writing an app that uses some image processing steps which I have implemented with Renderscript in order to bring processing time down to a reasonable level. I am completely happy with that part, thanks to the great support here.
Apart from Renderscript, I also make use of the Camera and both of these elements cannot be reliably tested on virtual devices alone.
I am targeting API 17+. I have a Samsung Galaxy Tab2 (API 17) and a Samsung S5 (API 21) but I want to buy 1 or 2 other (possibly older) devices in order to test on them. Of course I know that it is impossible to have a guarantee that it works on all devices, but what devices would you consider to be fairly "representative"?
You may want to try NVidia tegra devices and/or Google's Nexus series. I would recommend Shield Tablet 8 for Tegra and Nexus 6 or Nexus 9 for Google devices. You can look at CompuBenchRS benchmark performance on various devices before you make the decision to buy any of them (https://compubench.com/result.jsp?).